Engine Cutting out when Idling

[dogs_bolx]

Hi, this has probably been covered before, but looking for a quick answer as I'll be needing the car right for Tatton Park very soon!

My D has as of today, started to cut out every time the revs drop to idle. If I apply a tiny bit of revs it is perfect, when I release, it drops to around 500rpm and cuts out. This happens when both hot and cold and restart is no problem. The car drives perfectly when running until I drop the clutch to stop.

I have checked the micro switch on the throttle linkage and its clicking and driving a solenoid (I assume) that I can hear engage.

Is this likely to be an ECU issue (replace the chips in it?). The car has always hunted slightly on idle (usually more when cold). Is this also an indication of ECU problem?

Many thanks for any advice.

Martin.

[dmc4087]

Sounds like ECU, but I'm sure Martin will come along shortly with a proper answer. An owner here had a similar problem a few weeks ago - he would start the car, it would rev to 2000, then within 30 seconds or so the revs would slowly fall until coming to a stall unless he kept his foot on the throttle.

Replacement ECU sorted it.

[dogs_bolx]

Thanks for that. I've ordered the replacement chips as per the guide on the forum. I've ordered extra spare chips so if it works, I may be able to fix someone elses ECU too! I might replace some of the electrolytic capacitors too. Lukily I'm an electronic engineer and am comfortable (re)soldering boards and understanding logic/comparators etc.

I'll see what Martin has to say too before surgery.

Speak soon gents.

[stunned_monkey]

On the face of it it sounds like the ECU because nothing else can cause the engine to not have enough air.

It could be a mixture problem though. Do the following test: With the engine hot, take off the air filter and while revving the engine gently to keep it running, slowly come off the accellerator so that it drops to where it should idle. If you hear a kind of roaring sound coming from the air intake as it struggles to stay running, then the idle system is doing what it should - opening up to make the engine rev. if there's not enough fuel, it will do this but still stall.

If the revs just drop till it stops, then it's the idle ECU almost certainly

[dogs_bolx]

Hi Martin, good to hear from you. It's been a while!

I'll certainly try the hints you sent, sounds nice and easy!

IF it's the ECU, what parts do you usually replace in there? Is it the LM29** and 40** chips or more?

Many thanks, Martin

[stunned_monkey]

I "usually" send them to Rich H to sort out... often it's more than just the silicon bits that're a problem

[dogs_bolx]

Yeah, thats what I was thinking, plus maybe the caps as they can go out of tolerance over the years. I'll let you know progress.....

[dogs_bolx]

Hi Martin, I tried what you said, but couldn't quite hear the sound you where looking for. Most times though it cut out, other times it idled very low (but smoothing sounding) at around 400-500 rpm. I unplugged the idle valve and it rose the RPM a little, but not much difference. I have taken the idle valve out and tested it on a small 12v 1A PSU and it opens/closes very nicley. So much torque it jumps around on the bench! I cleaned it while out of the car. When engergised fully, its hard to blow down the inlet, and if I reverse the valve, its very free to blow down so I assume its OK.

Can you put me in touch with the guy you mentioned regarding ECU repair. I opened it up and nothing looks blown. Diodes test Ok with a meter.

Many thanks, speak soon.

[stunned_monkey]

RichH on here mate

[dogs_bolx]

Thanks Martin. What do you make of my tests above? Helpful? Does it give any more ideas?

[stunned_monkey]

Sounds like the ECU to me. I'm sure Rich will send it straight back if it isn't (he can test it on his own car). The valves don't often fail actually.

The problem for people like me to remote-diagnose anything is the multitude of symptoms these engines display for a given problem and it's often tough to know for sure without seeing it for myself. It might even be a vacuum leak, but it'd have to be a massive one....

[dogs_bolx]

Yeah I understand, thanks again. I'll let you know the outcome when it's resolved.

[dogs_bolx]

Ok, update as promised.

I shall post a full description of what I did shortly, and hopefully a Youtube video showing the de-soldering and soldering process I used to repair the ECU.

It was the Analogue/logic chips in the ECU causing the issues. I changed 5 out of the 6 chips as I ordered one incorrectly by mistake! Therefore I had a 1 in 6 chance of failure! However all is well.

Prior to changing the chips, I replaced the electrolytic capacitors and 3 power transistors. This didn't cure the problem but was worth a go!

If anyone needs advice, feel free to contact me.

[Guinney1971]

glad you sorted it Martin, and this sounds like an excellent write up for the Club mag
if you get 5 minutes (you can include a link to you YouTube vid if you didnt take photos).

So, you coming to Tatton Park then?

[dogs_bolx]

Yeah sounds a good idea, let me know how I can help or write it up.

Yes, should be attending Tatton! I Look forward to meeting other from the club.