Temperature & Electric Related Stalling Issue

Hello all my name is: Cristian
I have a: 1991 3000GT SL, Auto Trans

I’ve got quite a conundrum here, hopefully someone has experienced it before!

When the car is running and at idle (in any gear) it often surges and stalls.

From what I have been experiencing, I believe most (if not all) instances of surging or stalling occur at the first introduction of a large electric load (radiator fans turning on mainly) and/or shifting into drive or reverse. It also occurs when I slow down to a complete stop (stop sign, red light, etc.). The engine will experience a very steady surging while at idle until it inevitably stalls, I believe only if there is a load on the electric system.

The engine never stalls, surges, or bogs when the throttle is open even in the slightest. My current way to combat this stalling when coming to a stop is my giving the throttle just a hair of a touch at all times, even while braking or at a stop.

This issue occurs almost exclusively when the engine is warm and fully up to operating temperature. When I start it up in the morning on my way to work it runs perfect until a few miles down the road when its all warmed up.

So far I have:

  • Replaced the idle air control valve
  • Checked all hoses for vacuum leaks (none found)
  • Disconnected brake booster and plugged off orifices to check for a diaphragm leak (no luck here)
  • Replaced the PCV valve
  • Cleaned the throttle body opening (held throttle open and carefully wiped out build up with rag)
  • Just had a new timing & serpentine belts put on, along with spark plugs and ignition wires
  • Monitored voltage at battery while car was running and during a stall (12.6V Off, 14.1V Running)

I think I’ve ruled out my alternator and voltage regulator just by monitoring the voltage at the battery and watching it for fluctuations, looked all good there.

As I mentioned, the catalyst for the surging/stalling seems to be the radiator fans turning on. The fact that it only happens while I’m at idle is what’s really puzzling me here.

Side note - not sure if this is applicable at all but worth mentioning - my tachometer doesn’t work at all, like it just reads 0 at all times.

If anyone has any ideas I would love to hear them, even if they are just shots in the dark!

I know you said you replaced your IAC, but I replaced mine and thought it would fix my same problem and it didn’t. Turns out the one I bought was bad out of the box. Then once I got one that worked, I had too high of an idle because the previous owner only troubleshot the issue by adjusting the BISS screw rather than looking for the actual problem.

May be worth a try to check that one more time but that’s just my experience with a similar issue.

If it is an additional load that is causing the car to stall out at idle then it is not making the adjustment to increase idle air intake to compensate. In my mind the three culprits would be the IAC, the throttle position sensor, or the ECU.

What is your idle rpm with no load before the stall? Could be too low to start with.

Should be around 600, although I know some guys prefer it a bit higher.

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I’m not sure what my idle rpm is since my tach doesn’t work. It does seem pretty low but the cap on the BISS screw looks like its been on there for about 30 years lol, I doubt its been messed with.

I thought about the prospect that my IAC could be faulty out of the box, so I measured resistance across the coils of both new and old and they all looked good.

The ECU could definitely be a suspect, I wonder if the IAC gets “homed” prior to ignition by the ECU?

Now that I’m thinking about it, I bet it probably does. I’ll plug in my spare IAC tomorrow and check to see if it moves the motor at all.

Throttle position sensor is interesting, I’ll look into that one.


The ECU makes those decisions after getting the info from all the sensors and such. I’ve not experienced it myself as my ecu was replaced before I got it but I would say that it’s possible if not probable if it’s never been rebuilt or replaced that the capacitors could be leaking and causing it to react outside of its original parameters.

You’re definitely right about the not being rebuilt. This car was bone stock and pretty neglected at that.

I’ll have to check out the ECU then too, I would imagine that it’s pretty easy to see a leaky capacitor, fixing it is another thing though lol.

There’s plenty of reputable services that can do it for you for about half the cost of a replacement that may be faulty out of the box.

I would go for the ECU first. If the unit has never been serviced, I always start here because it can give so many weird faults that cost a fortune to try and solve.
Which country are you in? Could you add your country flag for us please.

I’m from the states, flag added :slight_smile:

I’ll try and get my ECU out and opened up.

If I do happen to find leaky capacitors, do I really need a rebuild service to repair them?

I may not be too familiar with how the circuit board works as a whole but I’m sure I could de-solder old capacitors and then re-solder new ones on there, if that’s really all there is to it.

Thanks for the input, Joe!

About 1 in 10 is a simple capacitor change but the board is very easy to damage if you don’t use the right equipment.
What starts as a simple capacitor change can end up being a right off if not done correctly.

Very good to know, I will refrain from trying a DIY repair then.

Is there any way I might be able to test the ECU in a broad scope to catch any common malfunctions? Or is this more of a rebuild for the sake of rebuilding something that is past its expected life cycle?

I’m not too familiar with testing protocol for electronics like this so I’d hate to start probing things and end up making things worse.

The ECU is way beyond its expected life span. Can you think of any electrical appliance in your home that is still working after 30 years?
A simple service now, will save a fortune in the future.
A lot of people make the mistake of delaying the inevitable.
Leaking capacitors will do damage to the board and other components. The longer you leave it, the worse the damage will be. It will eventually reach a point where it is irreparable.
If you have it serviced now, it should cost between $35 and $50 because there is no fault finding required. They only need to replace the main power rail capacitors. Once the ECU completely dies or has major capacitor leakage, you are in the realms of $200 and up.
As far as testing goes. The only simple test that can be done is to connect a scope to the main power rails within the ECU. This will allow you to monitor ripple on that rail. Any ripple will cause noise within the digital part of the circuit and that will be translated into corruption on the I/O lines which will give all sorts of random faults.

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Wow I didn’t expect a service to be so affordable! I will definitely do some research on where I can get it serviced.

I had the same problem and did every thing you have plus. After a year of this problem it was suggested I did a fuel pressure check, as this was a classic symptom of a failed regulator. No one including Mitsubishi even suggested this. Fault in my case turned out to be the fuel pressure regulator. Had no further problems.

I hadn’t thought about fuel pressure.

Is there a gauge hookup somewhere or did you have to rig something up custom? Seems like it should be too difficult to check.

What sort of behavior did you see when you checked it, just super erratic pressure or just low across the board?

The fuel presure tester is inserted in the fuel rail just above the coil pack. I bought a cheap tester from e bay for around £16 and also bought a second hand link pipe that joins the fuel rails on the cam wheel side of the engine, Removed the original then cut the spare link pipe and inserted the pressure tester between the two cut parts and fitted it to the engine Then ran engine.The FPR maintains your fuel pressure and decreases the fuel pressure while under vacuum (idle). As vacuum decreases when you press the throttle, your fuel pressure increases to help your injectors compensate for the increased fuel need. Typically your fuel pressure at idle will be ~38psi, and under WOT is rises to ~48psi (roughly 1 psi for every 2 inches of vacuum change). The manual has a write up with figures. The only fiddly bit was disconnecting the fuel rail at the cam wheel end and reconnecting. I used a universal gauge and adaptors which is why I chose to insert the gauge where I did. After I just put the old pipe back on. I bought a second hand FPR for around £15 and so far it’s been good for a year.


Wow that’s awesome, I’ll get myself a gauge and try it out for myself.

Thanks for the info!

I had a similar problem. Poor idle, driving along like a kangaroo at low revs, poor cold start, and I changed just about everything and it made no difference. I got the ECU checked out and it was like a new car it solved all the problems and its the best it has ever run in the14 years I have had it. Like Joe has said get the ECU checked out first and then you know you have a good base to start from.

Just to add to Joe’s words, I did my capacitor change myself and I can say I just caught them in time two of them had just started to eat into the board but not caused any faults. There is a kit available of the caps that leak or you can take the ECU apart and make a note of what you need. With a very careful bit of de-soldering the replacing them was the easier part of the task. Since that I have re-checked the board twice and all has been fine.

UPDATE: Month and a half later because I’m both busy and slow.

I just removed my ECU from its mount and popped the top to take a peek at what I’m dealing with, but it looks nothing like the pictures I’ve seen of leaky capacitors!

Honestly I’m kind of surprised how clean it is in there, being 30 years old. Speaking of which, ive seen some pictures where the metal casing is a gold-ish color, and some where it is gray. Mine is gray, does this bear any significance as to how old it actually is?

I’ll attach some pictures so the experts can have a look for themselves, as good a look as my photography skills will allow, that is.

As for replacing these capacitors, should I even bother? I have the replacement set here but to be honest I’m not 100% confident in my soldering abilities, don’t want to accidentally bump something and brick the board…

What do you guys think?

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I am with Joe on this one. Electeolytic caps of that vintage are already a failing component by definition. They are already leaky. And here I use the term ‘leaky’ in its original technical meaning - they start letting significant for that particulat cirquit DC thru, i.e failing to do their main function of blocking it. Which eventually start causing troubles due to them not enough smoothing out ripple on the rails as Joe mentioned.

When they start physically leaking out the electrolyte - that’s way past the point when they already failed electronically.