'91 500 SL - Idle Misfire

[jgl1]

'91 500 SL, 87K miles, with KE-Jetronic...

Have personally maintained this vehicle since new. Lent vehicle to family member for several months, during which time the alternator failed while the vehicle was out-of-state. Vehicle was towed to an M-B dealer, who replaced the alternator. Shortly there after, the belt tensioner failed, stranding the vehicle again. The vehicle was towed to a local repair shop, where the tensioner and belt were replaced, along with both ignition coils (to address difficulty in starting).

Once back in my possession, the vehicle presented with difficulty starting, a chronic idle misfire with idle speed of 650 RPM in Drive, the new style OVP relay that I had presonally installed a year earlier had been replaced with a unit dated "1989", the ignition control unit's retention nuts were missing and a different ICU was installed in the vehicle... correct part number, but not the unit that the vehicle was originally equipped with. There was no charge for ICU or OVP relay replacement on the service statement.

The following work was performed following receipt of the vehicle from the local repair shop:

1. Fault code check revealed a faulty oxygen sensor and sensor was renewed.

2. Fuel pressure was checked and found to be out of spec; fuel pressure regulator was renewed.

3. Spark plugs, distributor caps and rotors were replaced, on principle. Spark plug wire resistance was checked and found to be WNL.

4. Knock sensor wiring harness insulation had disintegrated and harness was renewed. Motor mounts were fatigued and replaced in conjunction with the harness renewal.

5. A new style OVP relay was installed.

6. Vehicle was subsequently driven for approx. 800 miles with fuel treated with LubroMoly Jectronic cleaner. Plugs were pulled and examined: 3 plugs presented with blackened, soot covered insulators. As such, all fuel injectors were replaced, along with accompanying idle air distribution hoses, seals, etc. Inspection of valves with injectors removed revealed essentially no deposits on the valves. Duty cycle was adjusted to 45-50% and the fuel mixture adjustment tower renewed.

7. Intake air and engine coolant temperature sensors specs were WNL but sensors were also replaced, on principle.

8. Cold start valve has not been inspected, as ambient temperature at start is consistently above 78 deg F.

9. EGR valve was inspected and found to be functional.

10. Idle speed control valve was removed, cleaned and found to be functional.

11. Air intake system was inspected for leaks using throttle body cleaner; no leaks were found.

12. Air pump has remained de-energized during the course of this repair, due to clutch failure.

13. EHA has not been inspected other than checking condition of pins and presence of current.


With the above listed repairs, the engine starts readily, runs well, and idles smoothly (but with the infrequent hiccup) except when starting the engine at ambient temperature... with the engine cold, the idle has a constant miss. The miss remains, regardless of engine temperature or duration of run - the run can last an hour and the idle miss persists. The idle miss also persist if the engine is shut down and immediately restarted.

BUT... if the restart occurs approx. 20 minutes after the 1st run from a cold start, then the engine idles glass smooth for the balance of the day, regardless of the number of restarts.

I don't have an oscilloscope to pursue this further should it be an ignition fault. Any suggestions as to what to inspect next? EHA? Ignition control unit (which apparently was switched out by either the dealer or the independent repair shop)? KE-Jetronic control unit?

Thanks for the input.

[riorust]

I would like to start with stating, I am no pro, but the symptoms you are describing match something that I experienced about a year ago.

You stated the ICU was replaced, but did not state a reason. When the ICU is replaced it is very important that you renew that heat transfer paste under it, some Indies just knock it out of the way. ICU's are heat sensitive and that paste does make a big difference in the ability for the ICU to shed heat to the fender well.

Hope it helps.

[Ferdman]

jgl1, I would track down the original ICU for your 500SL. From my experience with our daughter's 300CE the ICU's are very expensive (around $2,500) so I can't imagine any indie or MB dealer swapping one to help you without charging you. Something isn't right about that scenario. It makes no sense to swap the ICU's unless the other party wanted your good one ... sorry if I seem skeptical about the motive. Suggest you talk to the MB dealer and indie who worked on your 500SL when the family member had it.

As mentioned, the heat-conducting paste is critical to longevity of the ICU, as is maintaining the ignition system components (spark plugs, wiring, coils, etc.). Radio Shack sells it for computer applications.

[jgl1]

THere is no doubt that the car's original ICU and upgraded OVP relay were removed from the car. Whether the dealer or the independent shop did is unknown and is a separate problem; likely the vehicle was used as a parts donor car and the original parts were for whatever reason not reinstalled. It will be difficult to point a finger at this point since the two services occurred over 6 months ago.

The cost of the control units warrants confirmation of the defective status of the existing part prior to its renewal, so I'm at a bit of a standstill until I can obtain a unit of known quality to swap in.

Fresh paste was installed on the bottom of the ICU when it was removed for inspection. But it seems lack of paste would manifest itself as a potential probelm once the ICU was at operating temperature... which is not what is seen in this case: the vehicle misses when started cold; subsequent restarts when warm yield a smooth idle.

Tends to suggest an electrical problem, where something is not making contact when cold.

[dhjenkins]

Misfires are usually caused by a lean condition. It would be nice to know if it's happening on the same cylinder or randomly on all of them; of course, without a scope or misfire monitoring that's not going to be easy.

Once you've got it in a 'glass smooth' condition, I'd take the caveman approach and start tapping on things to try to induce a misfire. You could also take an upside-down can of air duster and spray it on things like the ignition control to cool them quickly.

Without the proper tools, it's sort of hit-or-miss.

[jgl1]

Agreed, without a scope this will be difficult to solve.

But perhaps, this could be looked at another way... ignore the ICU for the time being and assume that its good.

If so, then which of the remaining components that have not been renewed cause a problem such as this?

- Cold start valve, leaking fuel?

- EHA?

- Throttle position sensor?

I could replace these, as they are substantially cheaper than a new ignition control unit or a KE Jetronic CU. But could either the cold start valve or EHA, if malfunctioning, truly cause the symptoms described?

If not, then the only things remaining that could cause such symptoms would be either of the control units.. or an intake manifold vacuum leak.

Everything else has been tested and found to be functional: coils, plugs, distributor caps, rotors, fuel injectors, idle air hoses, oxygen sensor, plug wires... the valves were inspected during injector renewal and were found to be remarkablely free of deposits; by extrapolation, I assume that the fuel distributor is clean as well. Duty cycle is within range and no fault codes are present.

Any input as to whether a cold start valve or EHA renewal is warranted at this point?

[dhjenkins]

A leaking cold start valve wouldn't cause a lean condition, so it shouldn't misfire - if anything you'd run constantly rich.

TPS is probably not the culprit, because it wouldn't have any affect at idle - it would cause stumbling and such off-idle.

Could be the EHA, maybe not - have you tested the current to it?

The fact that it only happens on the first start-up of the day is very strange indeed.

When you say "...the engine idles glass smooth for the balance of the day, regardless of the number of restarts.", does this include re-starts where the engine has managed to get all the way back down to ambient?

[jgl1]

When engine temperature returns to ambient, the idle miss returns.

Checking the EHA later today.

[dhjenkins]

If the car only misses when the engine is "cold", I'd check the engine temp sensor wiring. The sensor (new, correct?) might work, but if it can't get its signal where it needs to go, it's useless. Are these engines equipped with an oil temp sensor as well?

[chicky]

Originally Posted by Ferdman

jgl1, I would track down the original ICU for your 500SL. From my experience with our daughter's 300CE the ICU's are very expensive (around $2,500) so I can't imagine any indie or MB dealer swapping one to help you without charging you. Something isn't right about that scenario. It makes no sense to swap the ICU's unless the other party wanted your good one ... sorry if I seem skeptical about the motive. Suggest you talk to the MB dealer and indie who worked on your 500SL when the family member had it.

As mentioned, the heat-conducting paste is critical to longevity of the ICU, as is maintaining the ignition system components (spark plugs, wiring, coils, etc.). Radio Shack sells it for computer applications.

Hello Ferdman,
Would you happen to know the part number of the MB heat-conducting paste?
Thanks in advance,
Chicky