'96 Ls400 - Cold Engine - Up/down Idle

[ck42]

Just got back from picking up the wife's LS400, after she'd gotten a ride home.

Got to the car and started it. Cranked fine.

Normal high idle (when engine is cold)

After a few seconds it dropped down to about 600rpm and then back up to a high idle again. Did this several times.

Somewhere in there the idle dropped to a 'burbling' 100-300rpm and felt like it might die....but would shoot back up again.

During one of those low 100-300 RPM idles, I tried pulsing the throttle to see if I could get the idle to come back up. It didn't seem to want to respond to my input.

Finally decided to see if I could hold the engine high enough to get the engine warm...to see if I was simply dealing with a cold motor related issue (something not telling the ECU that the engine was cold and putting it into an open loop condition).

Tried putting it into reverse to back up and the second I hit the throttle, the RPM's dropped to the point that I though it was going to stall. Let off the gas. Tried again. No luck. Put it in DRIVE...rolled forward on its own a little, but as soon as I tried to give it a little gas to go up a very slight incline, it wanted to die again. Kept trying and it did finally die.

NOW....tried to restart the engine several times. No go. WEIRD! Wouldn't turn over. Strong cranking though.

Wondered if maybe I got some bad gas. Opened the fuel cap for grins. Put it back on.

Got back in the car. Tried to start it again and it cranked fine. WEIRD!!

Idled a little high and no idle bouncing.

Waited a couple minutes.

Was able to get the temp gauge almost to a warm condition.

Put it in drive and drove around the parking lot a couple times.

Acted perfectly normal. Drove it home.

I've self-diagnosed several problems 'similar' to this, but the behavior in this case is a little unusual to me...and I've not ever had to deal with anything like this on THIS car.

Some guesses:

Bad gas

Coolant temp sensor

IAT sensor

TPS sensor

MAF sensor

ECU??

[curiousB]

Bad gas >>> Maybe but why all of the sudden? Easy to check, fill up from Tier 1 gas station

Coolant temp sensor >>> Maybe but usually fails to open circuit which means cold engine. Would typically cause engine to run richer than needed, lousy gas MPG but should run

IAT sensor >>>

TPS sensor >>>

MAF sensor >>> would probably trigger a code

ECU?? >>> Doubtful, often claimed but seldom is the problem. Some years had poor grade capacitors which dry out and need replacing but this is something to look at after ruling out everything else.

I would add to your list:

Problem ignition coil. common problem with these cars and you lose 4 cylinders when it happens. Maybe one coil is on the verge of failing

IACV and/or dirty throttle body. These guys get caked up and then impact idle performance. Cleaning is good routine maintenance.

[freegard]

I put my money on bad capacitors in the ECU. You are descibing exactly what was going on with my 94.

I replaced the 8 capacitors myself in the ECU and that issue and other issues disappeared.

One way to know if it is the ECU is to have someone read the codes. If the codes cannot be read or seem odd or incorrecvt it is certainly the ECU. The brain is malfunctioning and is not capable of diagnosis yet things are haywire.

[freegard]

It is NOT the igntion coil because it would run VERY rough missing on a number of cylinders; You have an idle problem not a rough running engine problem, huge difference. The engine would barely run and stall if one coil were bad.

It could be the Throttle Position Sensor or throttle body true. My bet is the ECU, especially if the car has lived its life in a hot climate. Heat causes the dialectric fluid to seep out ot the capacitors faster AND THEY LOSE THERE CAPACITY to moderate engine idle among other things. AND if it is true the ECU may throw a TPS code, however if after replacing your TPS the problem remains!! THEN ECU certainly!

[landar]

ck, you obviously have an intermittent problem and those are the hardest to diagnose. I would start your search with trying to separate the issue between fuel and spark. How do you do that? Well, when it wont start, try spraying some starter fluid into the intake snout and see if it begins to run correctly(if only briefly). Also, you might consider hooking up an OBDII scanner to your car. If you do not have one, you might borrow one or limp to an auto parts store where they will scan it for free. The scanners are fairly inexpensive nowadays. The scan is the first thing that I would do.

As I read your original post, I thought about fuel delivery, such as a weak or intermittent fuel pump causing the engine to starve under load. A fuel gauge on the injector rail would rule that out.

Let us know what you find.

[ck42]

All very good feedback guys. Very much appreciated.

This is the FIRST time ever this car has done anything like this. The only other malfunctions the car has ever had is a broken belt, broken timing chain tensioner, and two failed O2 sensors (at different times).

I've had a fair amount of self-diagnostic experience trying to diagnose a problem with a Ford that I had, so I learned a LOT at that time about all the various ways that the sensors work and how the engine behaves when certain things go wrong...so thankfully I'm not totally in the dark on this one. But, being the first time I've had to troubleshoot on this car, I certainly need some help from some veterans here.

I do have a ODBII plug-in diagnostic module (with the matching Android app called 'Torque') that will spit out real time diagnostic info and engine stats, so I'm hoping that I can get some useful info from it while it's acting up....as well as go out and read any codes that might have been generated (haven't done yet).

If it's the ECU and caps need to be replaced, no problem. I have YEARS of professional level experience working on electronics at the component level and replacing parts. For me, this would be an easy and fun fix.

[ck42]

Couple hours ago, car started but was running at about 100 rpm.

Just went out now to check for codes. No codes have been thrown.

Tried starting and it kicks over for a FRACTION of a second and immediately dies.

...does seem like a fuel related issue, but don't have any starter spray right now to use for a test.

[ck42]

Car is now stranded in a parking lot.

Behavior is now it will sometimes just crank and not turn over...sometimes it will turnover for a split second...and sometimes it will turn over and do a proper smooth running high-rev for about 2 seconds and then die. - and various iterations of these.

I'm really suspecting fuel pump or fuel filter at this point now, based on this latest behavior.

Where's the fuel pump located on this car? Difficult DIY? Guessing it's sitting in the fuel tank like most others and I need to pull the rear seat...

[ISRydr]

ck, although I have a 99 LS400, I've been going through the same issues for about 3 years now. Sometimes it would die when I stopped at a redlight. There were times the idle would be so low it would die when put into gear, other times it would only idle 300 rpm and at other times it would idle at 600 rpm which is still low. Never once was there an error code. Only recently did it begin to lose throttle all together and usually happened within a mile or so of starting up first thing in the morning. Everything would be fine then the check VSC / VSC off light would come on and the idle would jump to 1200+ and I would have no trottle the first half of the pedal. I began to notice a whirring noise that was coming from the throttle body motor, so I took it to my mechanice who checked the ohms and used a stethascope to determine the throttlebody motor was in fact going bad. I have recently purchased a good used throttlebody and am having it put on 2 days from now. My mechanic assures me this was the problem I've been having all along. I too thought for a long time it was the ECU but thankfully it seems to been elswhere. I will post my results here once it's been installed and ran a few days to confirm but at this point I think I've finally found the culprit. Don't overlook the TB motor as being the issue. Good luck with your search for the problem

[1990LS400]

so I took it to my mechanice who checked the ohms and used a stethascope to determine the throttlebody motor was in fact going bad. I have recently purchased a good used throttlebody and am having it put on 2 days from now.

How many miles are on this beast? Replacing the entire throttle body seems a little drastic if only the TPS is failing. The TPS is not a real expensive part when new. TPS failure is common on the 98-00 LS400. Here is a thread that includes me telling about how mine failed in 2008 at about 115,000 miles: