Mind-bending Rough Running...

[eatingupblacktop]

I was going to sugggest checking the crank and cam sensors, but you say that hitting the throttle smooths it out. A lumpy idle would suggest the IACV, but since you've done that, the only other thing that comes to mind is your coolant temp sensor. If it's sending erratic signals, the ecu will try to compensate by cycling through leaning out the mixture then enriching it.

[914lps]

Unless I missed it, why have you not replaced the fuel filter??????

[Alexander the o.k.]

And I may have missed it, but did you check the trunk hinge wire harness. This is a known problem which causes all manner of strangeness.

It caused me much heartache til I found the problem.

Maybe the repeated shorting at the harness forces the ECU to receive bad data, who knows?

In any event, it's easy enough to check, and costs $0 to fix.

It makes this smooth world class auto act like a balky misfiring lawn tractor with water in the tank.

Try it.

[designet]

And I may have missed it, but did you check the trunk hinge wire harness. This is a known problem which causes all manner of strangeness.

It caused me much heartache til I found the problem.

Maybe the repeated shorting at the harness forces the ECU to receive bad data, who knows?

In any event, it's easy enough to check, and costs $0 to fix.

It makes this smooth world class auto act like a balky misfiring lawn tractor with water in the tank.

Try it.

[Alexander the o.k.]

And I may have missed it, but did you check the trunk hinge wire harness. This is a known problem which causes all manner of strangeness.

It caused me much heartache til I found the problem.

Maybe the repeated shorting at the harness forces the ECU to receive bad data, who knows?

In any event, it's easy enough to check, and costs $0 to fix.

It makes this smooth world class auto act like a balky misfiring lawn tractor with water in the tank.

Try it.

I appreciate the quote... but was there anything else ?

[micadevcon]

Okay, update time. I know its an old topic but I'm still unable to find the source of the problem.

I had the fuel filter replaced - it was rusted on pretty good and my theory was that gunk inside settled when it sat for a while, and after running a bit would clog the filter up, blocking flow. Crappy theory and new filter didnt resolve the problems.

Got fed up and took the car to a shop to get some diagnostics done. They saw vacuum leaks at the hose from ISCV to the intake and at the EGR. I put a new gasket on the EGR, to no effect. Will replace the hose next. PCV hose is cracked and will be replaced as well - might have been what the tech's saw during their smoke test.

Checked the trunk harness, didnt seem to have any shorts? Left wires hanging as advised in the pertinent posting.

Checked the coolant temp sensor [one by the fuel pressure regulator] and it came up ok.

Other ideas: Could the intake manifold gasket be worn out? Engine is great cold but deteriorates when warmed up - possibly the gasket expands and leaks then? Also going to take a look at the TPS, as I am experiencing the hard-throttle, slow and jerky acceleration vs light throttle and good-smooth acceleration. Flooring the gas pedal results in hesitation and [what feels like] severe misfire vibration with slow increase in RPM. Lifting off on the throttle results in a sudden spike of power or at least faster, smoother acceleration.

My last idea is the ECU...?

[Bali26]

MAF? Sparks? Coil?

[micadevcon]

MAF? Sparks? Coil?

Already replaced MAF, Coils are good, tested against a new coil, no change. Spark plugs are good, there has been a problem with condensation buildup [NOT coolant] in the #8 cyl plugwell, cleaned that out but no change.

And if it were those things, why would it run fine when its first started cold?

[python]

have a fuel system cleaning service done, i had rough idling and had one done at the shop where i take my car,i think wynne's was used, but there are several good ones as well, like run-rite, its a type of I.V. system where it drips into ur intake and cleans out everything

[micadevcon]

have a fuel system cleaning service done, i had rough idling and had one done at the shop where i take my car,i think wynne's was used, but there are several good ones as well, like run-rite, its a type of I.V. system where it drips into ur intake and cleans out everything

I've done a full seafoam cleaning, cleaned out the throttle body and egr system. It cant be fuel related as the car runs great when its (been) cold (for a long time).

My idea now is that there is a gasket somewhere, intake or otherwise, that due to heat expansion of the engine, starts to leak when the car warms up. Anybody else had similar problems? Leaking intake manifold gasket after warming up?

[mejojo]

I have an active problem that sounds very much like yours.

I gave up and took it to Lexus dealer and they said that they could not diagnose it since I had aftermarket ignition parts (dist cap, wires, even the Bosch Platinum plugs would not allow them to proceed with diagnosis!!!.. Is this complete BULL or do they all say this?)

Mine started out with an occasional hiccup when going up an incline in cruise control...it would burp once and all would be fine.

A few weeks ago, (coincidentally?) a day after having to replace the accessory belt, the rough running sounds very much like yours, EXCEPT that when I step on the gas there is lots of power and the roughness is gone/hidden. The worst roughness is when idling in gear and all the way up to 60 in cruise control.

It is drivable but I don't really want to drive anyone else in the car that way.

I am at a loss and don't know where to go short of putting in $300-$400 worth of Lexus OEM ignition parts and taking it back to the dealer.

Joe

[mejojo]

I did some more tests...I disconnected the pax side coil wire and the engine idled inconsistently, just as it does when that coil is hooked up, (except on 4 cylinders.) I reconnected that coil.

I disconnected the driver's side coil and the engine idled quite evenly (though clearly on 4 cylinders).

Going further, I reconnected the coil and unplugged one spark plug wire at a time from the driver's distributor. When I disconnected the plug nearest the driver (is that #1?), the engine idled evenly, though clearly not on 8 cylinders...no jumping. Reconnecting that one, and back to jumpy idle.

This tells me (correct me if you have other ideas) that I have a bad (in order of suspicion):

1) distributor cap

2) plug wire

3) spark plug

The plugs are Bosch Platinum. The wires and dist are Autozone...(Duralast?) and all were replaced 20,000 miles ago.

If I had the energy right now to disconnect and reconnect the intake stuff, I'd go switch the plug wire with the another one as long to see if the problem follows the wire, but I don't so I'm not going to try that today.

I'd really like to get the shield off that's at the bottom half of the distributor cap, but I can only locate 2 or 3 of the necessary bolts. Is there a picture to help locate the bolts that aren't really visible?

Joe

[hlpyfix]

:o

I did some more tests...I disconnected the pax side coil wire and the engine idled inconsistently, just as it does when that coil is hooked up, (except on 4 cylinders.) I reconnected that coil.

I disconnected the driver's side coil and the engine idled quite evenly (though clearly on 4 cylinders).

Going further, I reconnected the coil and unplugged one spark plug wire at a time from the driver's distributor. When I disconnected the plug nearest the driver (is that #1?), the engine idled evenly, though clearly not on 8 cylinders...no jumping. Reconnecting that one, and back to jumpy idle.

This tells me (correct me if you have other ideas) that I have a bad (in order of suspicion):

1) distributor cap

2) plug wire

3) spark plug

The plugs are Bosch Platinum. The wires and dist are Autozone...(Duralast?) and all were replaced 20,000 miles ago.

If I had the energy right now to disconnect and reconnect the intake stuff, I'd go switch the plug wire with the another one as long to see if the problem follows the wire, but I don't so I'm not going to try that today.

I'd really like to get the shield off that's at the bottom half of the distributor cap, but I can only locate 2 or 3 of the necessary bolts. Is there a picture to help locate the bolts that aren't really visible?

Joe hi, joe first off they don't like bosch plugs if you can always use NGK that is the right plugs and it still could be a wire ? It could be arking in side the cylinder on the way to the plug or oil from the vavle cover leaking down in side check those and let me now hlpyfix

[hlpyfix]

:o

I did some more tests...I disconnected the pax side coil wire and the engine idled inconsistently, just as it does when that coil is hooked up, (except on 4 cylinders.) I reconnected that coil.

I disconnected the driver's side coil and the engine idled quite evenly (though clearly on 4 cylinders).

Going further, I reconnected the coil and unplugged one spark plug wire at a time from the driver's distributor. When I disconnected the plug nearest the driver (is that #1?), the engine idled evenly, though clearly not on 8 cylinders...no jumping. Reconnecting that one, and back to jumpy idle.

This tells me (correct me if you have other ideas) that I have a bad (in order of suspicion):

1) distributor cap

2) plug wire

3) spark plug

The plugs are Bosch Platinum. The wires and dist are Autozone...(Duralast?) and all were replaced 20,000 miles ago.

If I had the energy right now to disconnect and reconnect the intake stuff, I'd go switch the plug wire with the another one as long to see if the problem follows the wire, but I don't so I'm not going to try that today.

I'd really like to get the shield off that's at the bottom half of the distributor cap, but I can only locate 2 or 3 of the necessary bolts. Is there a picture to help locate the bolts that aren't really visible?

Joe hi, joe first off they don't like bosch plugs if you can always use NGK that is the right plugs and it still could be a wire ? It could be arking in side the cylinder on the way to the plug or oil from the vavle cover leaking down in side check those and let me now hlpyfix

the dist. cap is not lucky the problem wire or pulg

[micadevcon]

It could very well be the plug. Mine originally had Bosch Platinum +4's, and ran okay, but ran better with NGK Iridiums. Try swapping the plug wire with the corresponding cable on the other side of the engine, and disconnect again to see if it fixes, and that may tell you if its the wire or not.

On the topic of mine, I think I have come full circle. When I got the car the first thing I changed was plugs and wires - one wire was severed! Engine still ran oddly - but wasnt shaking as bad as it is now. But I do recall that in troubleshooting a TRAC light, I replaced the O2 sensors with Bosch universal O2 sensors... And then she started idling rougher - I didnt make the connection at first. So, combining the IAC adjusting itself past perfect mixture and temperature-related, it must be a sensor problem. And per http://www.babcox.com/editorial/ic/ic10044.htm, Bosch O2 sensors produce more hash than Japanese sensors, which could very well be confusing the ECU on this poor '91.

I have ordered a couple of Denso O2 sensors to put in, and will let you all know what happens.

[300zee]

I've got the same exact symtoms on my 97 LS400 with 175k on it. Have you ever checked the fuel pressure? You did have a "gunky" filter right? So this in turn would cause the fuel pump to work a lot harder and cause it to fail sooner. I haven't checked my pressure yet, but that is the first thing I'll do as soon as I find an adapter to the fuel rail. My car had a CEL on and the codes were P0300 random misfire, p0302, p0304, p0306, p0308 Cyl 2,4,6,8 misfires and also a Bank1 system too lean. Now this shows me that the engine either doesn't have enough fuel or I have an intake leak. Now I removed the vacuum line off of the fuel pressure regulator and this should increase the fuel pressure and make the engine run rich, But I noticed no change. Most cars you will notice either an increase in RPM's or chugging because it is running rich. The next thing I tried was to pinch the return fuel line. This would dramatically increase the fuel pressure to 60 to 80 psi, so you only do this for a couple seconds at most to check pressure or engine change otherwise you could damage the fuel pump. So when I did this there also was no change in engine idle. So my next step is to check fuel pressure and confirm my theories. The only part that does not fit into my theory is that the problem happens when the engine idles for a long time and warms up. This makes me think of an intake leak, but I'll test the fuel pressure first, and when that is good then I'll look into others.

I read all you posts and noticed you replaced a lot of parts. How much did all those parts cost you? Are they all new parts or just more used parts that are possibly bad as well? Did you test all these parts before you replaced them or are you just shotguning parts at this?

[micadevcon]

Fuel pressure is still a slight possibility, still wouldnt explain how she runs perfectly with a manually adjusted IAC, or when cold... If the O2s dont do it then thats the next step.

I havent been able to test fuel pressure because of the odd fittings used [banjo-style with center-drilled bolts], which I cant find the dual-hole bolt that you can put two fittings on anywhere to test with, as you are aware of...

I've put in about $1000 in parts, but there were some other parts not related to this issue that needed replacing [HVAC Control, Alternator, Hood Struts, Idler Pulley, etc]. And a few were related and needed to be replaced [Fuel Pressure regulator was completely blown, replacing O2 sensors did fix TRAC light error]. I didnt go with brand new parts, or my cost would be easily 10x as much. I got some salvage parts, others on ebay, and local shops - and the vast majority of the parts I replaced were tested first, to the best of my knowledge - which wasnt much at first! ;) But I made sure the replacement parts were good quality too, proper ohms, etc. And I didnt skimp on the criticals [rotors, finally got OEM wires, got best plugs, toyota fluids].

After a shop tossed out my IAC hose [split], I had to make my own with regular hose. Being unformed, Its squeezable, so when it started to stumble at idle, I disconnected the IAC so it couldnt compensate, and squeezed the hose a bit, and she started to die! This tells me that it isnt a lean problem, as if it were leaking elsewhere, reducing the airflow through the IAC should even it out - and it didnt. MORE airflow is needed. Thats why, when I adjusted the IAC myself - full open - and started it up. while it wouldnt start the first few times, once it held, it idled PERFECTLY, but slowly restored itself to poor running condition. This tells me that its sensor-related, as something is telling it to close erroneously, past the point of it running smooth. So, the O2 sensors have to be giving it some erroneous or confusing [hash] signals. Otherwise wouldnt it see that it is running perfectly and stay there with the IAC?

I have checked and re-checked, and she isnt giving me any codes.

[jcrome04]

Have you EVER taken it to a CERTIFIED LEXUS TECHNICIAN ???????????

[micadevcon]

Have you EVER taken it to a CERTIFIED LEXUS TECHNICIAN ???????????

The closest Lexus Dealership is 2 hours away and I simply cannot be without my car for god knows how long. Not to mention I really cant afford paying enormous hourly rates and ending up paying gigantic OEM part fees. NO sir, not when a stinking MAF is $1700, an ECU is $1350, and an IAC is $700. I can only imagine what the hourly rate is. And please dont bugger me on "You shouldnt buy a Lexus if you cant afford to fix things." This is a Lexus. Things arent supposed to need fixing so much. Ive owned three fords, a dodge, and a volkswagen with less problems. (most of those cars OLDER than this one) I saved up (Yes, $3000 is alot to a young adult NOT making $50,000+ a year) and bought this car for its "reliability", assuming its 'quirks' were easily fixable. How wrong I was.

But I am keeping up hope.

If I had a dealership nearby and they could tell me what is wrong in an hour or two or labor, I'd do it. But that isnt my situation, so I have to do what I can. I like to work on my own cars and throwing money at a dealership for a car 17 years old doesnt help me down the line when something else goes wrong or needs fixing. Experience is a great teacher and I wouldnt trade this time learning how every part of my car works for spending thousands at a dealership.

And if this forum is going to be all "Take it to a dealership" then very well, I'll figure this out on my own. I just thought that others might be interested in fixing similar problems instead of throwing in the towel.

If the opportunity presents, yes, I would like to take this car to a Lexus tech - but I dont have that convenience.

[mejojo]

1) I find dealer service departments to be lackluster, at best...ask me to tell you my Volvo story some time.

2) As I said in my recent post in this thread:

I gave up and took it to Lexus dealer and they said that they could not diagnose it since I had aftermarket ignition parts (dist cap, wires, even the Bosch Platinum plugs would not allow them to proceed with diagnosis!!!.. Is this complete BULL or do they all say this?)

Joe

[300zee]

Update...

I have the adapter to check fuel pressure at the fuel damper spot. 10 psi. Replaced fuel filter which was very dirty and black. Still no change in pressure. Replaced the fuel pump, as I had suspected from the begining, and what do you know ---- The car has 38 psi at idle and 44 psi at full throttle, and purrs nicely. DONE

So if you want to stop by my shop I could show you how to measure the fuel pressure. Today's lesson. Whenever you start diagnosing a problem, check your basics first. Such as fuel pressure, air intake leaks, spark plugs. I spent less than $200 on the fuel pump and filter and nothing on unneeded parts.

If you need any repair work done call me 612-325-5912 (Paul). I am just opening my new shop. Wish me luck!!! I quit my dealership job today.

[oldskewel]

Have you EVER taken it to a CERTIFIED LEXUS TECHNICIAN ???????????

The closest Lexus Dealership is 2 hours away ... enormous hourly rates and ending up paying gigantic OEM part fees. ... I have to do what I can. I like to work on my own cars and throwing money at a dealership for a car 17 years old doesnt help me down the line when something else goes wrong or needs fixing. Experience is a great teacher and I wouldnt trade this time learning how every part of my car works for spending thousands at a dealership.

...

That's the spirit!

This sounds like a really tough problem, and you already know you've replaced more than you needed to, but the learning experience is invaluable. Especially when you take the effort to get good diagnostic tools and think carefully about the problem as you are clearly doing.

Hope the fuel pressure test / fuel pump replacement fixes it - that sounds promising.

I have 3 degrees in mechanical engineering from the two best schools in the US (luckily, all was free), yet some of the best engineering/problem solving education I've had has been from fixing my cars. This started with my 1979 FIAT spider that I got in 1986. I owned it for 11 years, self-taught myself everything while replacing everything but the distributor and starter (I did engine rebuild, body work, etc.), and sold it for $700 more than I paid for it.

A friend of a friend who is a space-station astronaut has said that despite all the education (Ph.D.) and astronaut training he has received, the most valuable skills he takes up into space have come from the many hours he spent tinkering with his single-engine airplane.

So keep learning, and thanks for sharing the story.

Ed

[micadevcon]

300zee, Thats great that the fuel pump fixed your issue! I will be stopping by St. Paul next weekend, and if my O2 sensors dont fix the issue, I just might arrange to stop in. I do recall hearing odd noises from the back of the car [fuel pump presumably] when turning. Plus when shes idling rough theres alot of clicking from the passenger side - I'm thinking either the ECU is making adjustments mad fast or something to do with the fuel pump relay. Just another possibility.

About the basics, yeah, I hear you. For me it was hard to grasp the basics on this car at first, not really understanding how it all worked together, but now have a much clearer view on its operations!

And thanks oldskewel, that helps alot as well - experience pays off in the long run. And I hope to continue gathering it!

O2's should be in any day now...

[micadevcon]

Well, got frustrated not finding the union bolt for the fuel pressure test, so my father and I decided to make our own.

We took a single union bolt from a junkyard fuel rail off a 1UZFE, drilled the top out, tapped it with 1/4" npt fitting to 1/8" fitting for our fuel pressure gauge. Got it all fitted together, tightened up and solid. Put in on the end union next to the pulsation dampner and attached our gauge.

Started the engine up per the tech manual, 35 psi. Disconnected regulator, 44psi. Fuel pump works great, so that is not my issue. I have verified that my Fuel pressure VSV is broken, no continuity and does not switch to high pressure above idle. But I have confirmed that the VSV not switching isnt causing the problem, as I disconnected the vacuum line, plugged it, and test drove it, still same symptoms. So, I'll find a VSV to replace it, but the change it makes isnt enough to be causing my idle problems.

O2 Sensors still havent arrived, anxious to get them! :D Will update after they are installed...

[mejojo]

Well, my issue turned out to be just a spark plug that would misfire about 5-10% of the time. In all my years I can say I've never had (until now) a spark plug fail on me. This must have just been a bum Bosch Platinum. Sorry to throw in here with one that turned out to be so easy, but I have had this sucker "down" for weeks trying to get it solved, thinking it had to be something else.

Sorry to have butted into the thread...and BEST of luck with yours. I'll check back to see how you've come out.

Joe