Weird Throttle Issue With My '94 Ls

[Ross W.]

Hi all

This has been exasperating my mechanic and I for quite awhile now. At approx. 55 mph, if I lift my foot off the throttle slightly, the ECU will shut the injectors off, as it would with a closed throttle. If I hold my foot at that point where it happens, the car will coast down to approx. 50 mph, then it picks up again and maintains that speed. When I advance the throttle again, the car shudders, bucks, and heaves, sometimes almost violently. According to my mechanic, the TPS is within spec and adjusted properly, as is the throttle linkage. The ECU has no fault codes stored, pending or otherwise. The ECU failed early last year and was replaced with a new one, but the problem has not gone away. There are no vacuum leaks, and the IAC is working fine, as are all the sensors. They've all been checked. This year the EGR system was replaced, but again that did not change the issue. All electrical connectors are clean with no corrosion. Plugs and wires I changed at 180k; it has 202k now, but again replacement made no difference. Upstream O2 sensors were replaced with the ECU. This happens with the cruise control too; sometimes the safety override kicks the cruise control off. The problem also gets worse over time when on a long drive. When I smogged the car this year, it passed with flying colors. .1 ppm nox; zero on everything else.

Folks, any ideas? :(

Thanks in advance & regards,

Ross

[steve2006]

Hi,

Sounds like you have a really strange fault there.

You say the TPS has been tested and set up correctly but has your mechanic looked at the live data on an oscilloscope to ensure the voltage output is steady with no glitches indicating a "noisy track"

If you ever had an old radio at home you may have noticed the volume control sometimes produces a crackling noise when adjusted due to a worn electrical track within the control, the operation of the TPS is very similar and it can suffer from the same problem. I believe there is also a microswitch within the TPS which should only close at idle speed this could also be acting up.

Thinking about it 55MPH is that not the cruising speed on your freeways and therefore the TPS will be the most worn on the track that corresponds to that speed.

It may be worth replacing it if this check cannot be performed.

Does the problem occur at 55MPH whichever gear you are in i.e. 55MPH in 3rd and 55MPH in overdrive?

Have you had the fuel pressure checked? Is the fuel filter ready for a replacement?

A long shot here but have you checked all the engine bay earth connections as loose and/or corroded ones can cause some very strange faults and finally the old chestnut the trunk hinge wiring loom this causes a lot of problems and even though I haven't heard of it causing your fault it may be worth checking.

[Ross W.]

Hey Steve!

To answer your questions, yes the mechanic has watched "live" with his scope, and has observed the injector shut-down on the scope/scanner, while on the road. He's also examined the TPS for any dead spots, and there are none. And it was replaced in '04. Not sure about the microswitch inside it. Original fried wide open and the ECU did not pick up a fault code for it! Fuel pressure is dead on, and the filter was changed three oil changes ago. 55 mph is the speed limit on secondary roads here; freeways are 65 or 70 mph. This issue happens approx 55 mph with the overdrive engaged, approx 45 mph o/d turned off. Ground leads, at least that I could reach, are clean. Trunk wiring loom sounds unlikely, but I'll check that out.

Thanks!

[curiousB]

Hey Steve!

To answer your questions, yes the mechanic has watched "live" with his scope, and has observed the injector shut-down on the scope/scanner, while on the road. He's also examined the TPS for any dead spots, and there are none. And it was replaced in '04. Not sure about the microswitch inside it. Original fried wide open and the ECU did not pick up a fault code for it! Fuel pressure is dead on, and the filter was changed three oil changes ago. 55 mph is the speed limit on secondary roads here; freeways are 65 or 70 mph. This issue happens approx 55 mph with the overdrive engaged, approx 45 mph o/d turned off. Ground leads, at least that I could reach, are clean. Trunk wiring loom sounds unlikely, but I'll check that out.

Thanks!

Is fuel pressure "dead on" at highway speed when more fuel is being consumed or just dead on when idling while parked. I’m just wondering if there is some fuel starvation going on at higher engine load. The fuel pump isn't very "taxed" at idle speeds. Maybe unlikely but if you could measure fuel manifold pressure while driving (tape gauge on lower part of windshield facing inside and go for a ride) it may confirm or rule this out.

I assume the ECU would log an error code if the TPS had a dead spot and since you don't have errors it doesn't seem to look like a worm out TPS.

You say problem is around 50MPH though. What if you get on an interstate at say 65-70 MPH is it all good at higher speeds? That would tend to rule out the fuel pump theory I suggested earlier.

Are you certain its not ignition? Coils bogging down at higher RPMs?

Sounds like a strange one. Good luck.

[Ross W.]

Is fuel pressure "dead on" at highway speed when more fuel is being consumed or just dead on when idling while parked. I’m just wondering if there is some fuel starvation going on at higher engine load.

Fuel pressure & flow check out. If there were an issue like that, a lean code would trigger, and it would be happening all the time, under load. As in my original text, it happens right about 55 mph, and if I lift off the throttle very slightly, the ecu shuts the injectors off like I'd got right off the gas. If I hold my foot right where it happens, the car will slow to about 50 mph, and then picks up again. The scan tool shows this happening (injector pulses flatline), but there's so far no reason for it. It only happens in this narrow speed range; not above it or below it. Years ago when I first had the car, the TPS went bad like I'd mentioned, and it was replaced under warranty, but the warranty mechanic installed the new switch backwards, and that created a similar problem to what I have now. But that issue was a lot more pronounced, and it wasn't speed related like this issue is. I went to a private mechanic after they refused to do anything about it, and he replaced the TPS yet again to eliminate the risk that the other one was damaged during it's butchered install. This is the one still in the car, and it checks out fine. No dead spots or glitches; a smooth impedance change from idle to full throttle.

Yes, a strange issue indeed, and very frustrating.

[jpourcy]

Try this:

http://us.lexusownersclub.com/forums/index...showtopic=42998

It sounds like the same problem that was on some 95-96 models. Off Throttle Shock.

When you changed your ECU was it brand new or used or rebuilt?

Jerry

[curiousB]

Is fuel pressure "dead on" at highway speed when more fuel is being consumed or just dead on when idling while parked. I’m just wondering if there is some fuel starvation going on at higher engine load.

Fuel pressure & flow check out. If there were an issue like that, a lean code would trigger, and it would be happening all the time, under load. As in my original text, it happens right about 55 mph, and if I lift off the throttle very slightly, the ecu shuts the injectors off like I'd got right off the gas. If I hold my foot right where it happens, the car will slow to about 50 mph, and then picks up again. The scan tool shows this happening (injector pulses flatline), but there's so far no reason for it. It only happens in this narrow speed range; not above it or below it. Years ago when I first had the car, the TPS went bad like I'd mentioned, and it was replaced under warranty, but the warranty mechanic installed the new switch backwards, and that created a similar problem to what I have now. But that issue was a lot more pronounced, and it wasn't speed related like this issue is. I went to a private mechanic after they refused to do anything about it, and he replaced the TPS yet again to eliminate the risk that the other one was damaged during it's butchered install. This is the one still in the car, and it checks out fine. No dead spots or glitches; a smooth impedance change from idle to full throttle.

Yes, a strange issue indeed, and very frustrating.

I wonder if it would be helpful to try an experiment. Intentionally slightly mis align the TPS so that when you are at 50-55MPH on the hwy the TPS will be in a different zone of the potentiometer sending a different voltage to the ecu. I know this won't fix it but presumably if it is TPS related the certain spot of operation (ie voltage out to ECU) this might shift up or down in speed with the alignment/linkage shift. If the car still bogs down at exactly the same speed as before maybe the TPS angle is a red herring and it is more to do with the ECU in isolation (or maybe ignition but seems less likely). Seems like it would be simple enough to try. At this stage just getting more data points is useful.

[Ross W.]

Intentionally slightly mis align the TPS so that when you are at 50-55MPH on the hwy the TPS will be in a different zone of the potentiometer sending a different voltage to the ecu.

I may just try that. The ECU, the transmission, the ignition system and so far all the sensors have been ruled out. There may be an issue with the TPS that a multimeter or oscilloscope can't read.

Thanks!