R129 with M119 - Induction Leak?

[E55BOF]

The car is a 1994 R129 SL500, with LE-Jetronic fuel injection. If you've seen my other thread, yes, that R129, but I think I'm getting there now.There is still a bit of a misfire, despite the engine loom having been renewed, but I dare say that will clear with a good run. The car has done two - yes TWO - miles since MoT in February, so all its running since then has been on a rich or very mixture.

The problem I need help with is that as the coolant temperature rises, it starts to run really roughly, with increasing amounts of throttle needed to keep it running, and eventually it cuts out anyway, even from 2500 rpm (all this is stationary, and in Park, by the way), and will not restart until it has cooled down a little. My suspicion is an air leak on the induction side, after the MAF, which becomes progressively more significant for the mixture as the automatic cold start enrichment reduces.

I doubt I'm the first to have this problem; can anyone give me a clue?

 

[John Jones Jr]

Worth throwing a new (ECU) engine temp switch on it anyway just for sake of a few pounds?

 

[hotrodder]

Induction/vacuum leaks usually cause different symptoms... when the engine is stone cold fuel doesn't 'burn as well' so a little extra unmetered air has less of an effect or results in a poor/rough/low idle depending how bad the leak is. As the engine warms up and the ecu starts listening to feedback from O2 sensors etc (for closed loop running) the extra unmetered air causes an increase in idle speed as the O2 sensors are reporting a weak mixture so add a bit more fuel. At the same time the ecu has a target idle speed so typically you get hunting as the ecu is trying to correct both mixture and idle speed but can't due to the vacuum leak

As said could be a coolant temp sensor amongst various other things. Any fault codes? Have you pulled any plugs to see if they're sooty etc? If it's drivable when cold then personally i'd take it for a spin up & down the road or around the block rather than leave the thing idling until it starts misfiring/the misfiring gets to bad to keep it running

 

[E55BOF]

Thank you very much for that; you know more than I do. Still early days; haven't got to the plugs yet. It's not drivable; I daren't risk it conking out...

The engine is a 119.972. It won't idle from cold without throttle for more than a minute or two, and it runs roughly with the idle hunting between about 500-850 rpm, then it cuts out.

I was running it before holding the revs at 1500, then 2000, then 2500 on the throttle; with each increase in revs, it warms up a little more/runs a little longer before it cuts out.

I'll try the coolant temperature sensor first; it's cheap enough. Boy, I do hope that's it!

For the codes, I have a cheapo-cheapo generic code reader I was given as a present, with a 16-pin plug; could you tell me where the port to connect it is?

 

[Doodle]

The port is at the back of the engine bay, passenger side, under the removable panel.

16pin implies OBD2; if so, it won't work. At this age it's pretty much STAR only.

 

[E55BOF]

B****r!

Let's hope the coolant temp sensor does the trick, then.

 

[John Jones Jr]

Let's hope the coolant temp sensor does the trick, then.

It very well may not but at 20+ years old it's still worthwhile replacing imo.

 

[Vlad]

Just also double check under the air filter body. When that gets taken off, its easy to disrupt some vacuum pipes. I did this on mine inadvertently and it ran like a complete dog. Popped the hose back on and she ran like a dream.

Worth a check

My main worry on your car is that when the wiring loom went it spiked the ECU.

 

[E55BOF]

Nothing loose under the air filter; I checked.

The loom was changed as a precaution; so far as I know it never went wrong.

The temp sensor is, I presume, the one with a blue plastic body and two large pins just to the right of centre, top front of the engine as looked at from the front?

 

[E55BOF]

Terry at Wayne Gates (who is sorting my E55K as I type) suggested that as the car has had plenty of jump starting, checking the OVP relay/Base Module would be a good next step. As usual, I can't find it on WIS...

Worse, I can't find the b****y OVP relay/base module either! Unless I've gone selectively blind, it's not under the black plastic cover left rear of the engine compartment, and it's not in the relay compartment behind the fuses right rear of the engine compartment either. Could some kind soul tell me a) whether the 1994 model R129 has the relay or the module, and b) where I should look for it? Please help...