Idle Surging

[Publius]

Hi,

Searching for some sage advice.
The '95 E420 I've had has developed a surging/high idle.

To get a perspective, it has had:
Fuel Pumps, FPR (Bosch units);
Wiring harness replaced by PO;
Thermostat and temp sending unit;
All new ignition system, including non-resistor spark plugs and new dust shields

I just replaced the ETA with a unit manufactured in 2004.

After this replacement a surging idle developed as it gets warm. I checked vacuum lines visually and all apear to be alright. Although when pulling the line to the EZL, the surging stops.

Could this be an EZL issue?

 

[gsxr]

Was the idle OK prior to ETA replacement, and the high/surging idle started with the 2004 ETA? Also, does your E420 have ASR or not?

In general I'd check for a vacuum leak via smoke test, or at least trace all the PCV tubes to both valve covers.

It's pretty unlikely to be the EZL at fault but you'd need either a spare EZL, or live data from the existing EZL, to confirm.

I'm assuming you checked fault codes, particularly on the LH, EZL, and either T/LLR or E-GAS modules (for non-ASR and ASR cars, respectively). There may not be any codes but it doesn't hurt to check. There can be fault codes despite the CEL being off.

 

[Publius]

Idle was at ~800 rpm, but idle would drop when engaging into gear but back to 'normal'--I assumed this was the faulty wiring of the ETA.


It is a non-ASR model, I did replace the T/LLR model previously.

I will check for codes.

EDIT:

I checked for codes:

All clear except,
Pin 7 Code 3
Pin 19 Code 6- Bad ISC? Is this the T/LLR module?

 

[Publius]

Turns out both the original and the new 'used' replacement T/LLR module that controls the idle are not working properly.

The culprit is most likely the short circuit upstream in the wires of the ETA.

So do not replace the T/LLR module before replacing the ETA!!!

 

[Ascension]

Just had this exact same issue with my 95 E420 without ASR. First thing I noticed was the cruse suddenly stopped working then a high idle and surge in idle when it got warm. Assumed my ETA was going or had a vacuum leak but tried a different T/LLR and it instantly cured the issues.
If you have a non ASR 400 or 420 with these symptoms don't immediately just assume it's the ETA or a vaccum leak like I did. My issues emerged soon after I installed a used updated ETA so--.

 

[natejgreene9871]

Just had this exact same issue with my 95 E420 without ASR. First thing I noticed was the cruse suddenly stopped working then a high idle and surge in idle when it got warm. Assumed my ETA was going or had a vacuum leak but tried a different T/LLR and it instantly cured the issues.
If you have a non ASR 400 or 420 with these symptoms don't immediately just assume it's the ETA or a vaccum leak like I did. My issues emerged soon after I installed a used updated ETA so--.

Ascension/Publius, I have a 1993 400e (non asr) I just bought and found this thread.

I have a similar issue I am troubleshooting. Car wont run correctly with EZL vacuum line plugged in. Runs fine unplugged.

So your issue was with the T/LLR ? Does my video look to be similar to the issue you were having?

 

[Ian White]

Mine had a similar issue, surging/hunting when hot. Mine has a bad witting harness I am currently in the process of doing the intake manifold and the wiring harness.

When I unplugged the vacuum hose to the EZL, the idle stabilized. Car is a non ASR model, so perhaps it is the T/LLR.

Side note- what does EZL and T/LLR stand for?

 

[gerryvz]

EZL - Elektronische Zundanlage

 

[gsxr]

T/LLR is the throttle control module for non-ASR cars (idle speed control + cruise control)

E-GAS is the throttle control module for ASR cars (idle speed control + cruise control + electronic throttle control)

The two modules are NOT interchangeable.

 

[Trae]

Hi guys. Been enjoying driving the 1992 Renntech 500E lately. Everything smooth as butter until I decelerate, at idle, through 20mph and slow to a stop sign.

The RPM will surge between 500-1,000 RPM until the car comes to a stop. THEN, it returns to normal.

If I barely touch the accelerator pedal, surging disappears.

NO vacuum leaks via a smoke test.

Have not tested codes yet, but will this afternoon.

Any thoughts? Merry Christmas!

 

[gsxr]

Trae, has the ETA ever been rebuilt or replaced with a rebuilt unit?

 

[Trae]

Don't believe so. Will check records.
Everything runs great with no Iights illuminated.

Did not have a chance to run blink codes though.

 

[winmutt]

Does the surging idle only occur when in gear? This is my symptom. I've swapped out a number of LH modules and the T/LLR module did go bad a year or so back, but I've been fighting this since day 1.