Heat soak revisit

I know we spent a lot of time a few years ago about hot soak start/idle issues. I am still having issues, which seem to have worsened since i switched out my plastic gm vacuum advance valve (that three line part that supplies vacuum at idle to distributor vacuum advance) to a lower vacuum setting, thus retarding timing a few degrees at idle.

But instead of focusing on fuel vaporizing in the rail, I’m focusing on GM ignition module.

A test that I did that actually allowed it to idle pretty close to normal upon hot start was to connect vacuum line straight to vacuum advance. This advanced timing quite a bit at idle. This isn’t fixing problem. But I’m wondering what it could mean.

I then hooked things back up to normal, but sprayed some water on top of my ignition module box. The idle wasn’t as good as advanced timing, but it was much better than before.

I do know opening hood after parking fixes the hot start issue. I need to test cooling the module box with a few more test drives.
But possibly a relocation of GM module is finally at hand for me???

Can an overheated GM module cause some kind of timing signal error to the ecu??!!

Those GM modules would act up with age, and heat was always a thing. Generally they fail dead as in no start and aren’t hard to test but I’ve heard of them causing a rough running condition but never seen it. I suspect it’s difficult to get a quality replacement these days. In the HEI application they could vary dwell time but actual ignition timing was still handled by vacuum and centrifugal advance. I don’t know how Jag implemented them but I’d be about the same way. I’d be surprised if finding quality made replacements isn’t a thing at this point. The parts store ones were hit or miss twenty years ago, GM still sold them though.

Your problem still sounds like fuel related heat issues, lot of XJS’s seem to have them. The fuel system isn’t the best thought out thing. Or it’s over-thought out might be more accurate. But if your timing isn’t right that will cause all manner of grief, bets to put a light on it and verify by the book.

I think the inlet manifold is already about the coolest part of the engine.

What did you have in mind?


We went through this a few years ago with Kirby. You’re right, it is the coolest part while engine is running. But after shutdown for 30 minutes, it becomes a great heat transfer.

I know Dave relocated his to firewall.

The next thing I’ve tried and will test tomorrow is redoing the thermal paste. I think last application i smeared way too much, and tightened down too hard which squeezed a lot of paste out. This time a thin layer on both sides, and only finger tight.

Perhaps it’s fuel AND ignition module. I can’t redesign the rail, which Bernard talked about at one time. But i can try to cool ignition module.

Last ditch effort is to treat the symptom…I would hook up an electric valve to my 45 second timer at start to temporarily advance timing with full vacuum to vacuum advance at distributor.

Just for fun, here is my approach, in no particular order-
Amp has a good sized heat sink, and a computer fan hooked up to brake switch.The amplifier box has been opened up so air can flow through it.

Ducted cool air comes from front of car, with insulated 2inch hose.

Hood has louvers to let heat escape. (Makes a BIG difference.)
Temp of module is monitored with dedicated temp gauge.
Never a problem. I’ll be interested to see what you do next!
Forgot something- The small electric fan is also controlled by an adjustable piggy-backed thermostat sensing the air temp so that it turns on at 160 degrees, which usually takes about a half hour of heat soak


I know your GM module is new, but maybe it’s failing?

Not the problem.
What the thermal paste does is to fill all the voids between the two surfaces with heat conducting material instead of air.
What you did the first time was good, finger tight I think will be worse.

Maybe a good idea. Let us know if it works.

You could also try bypassing the fuel relay, prime the pump for 10 sec, start and see if there is a difference.
I did it and generally the engine starts easier.
Easy to implement permanently with a delay relay, and maybe you could combine it with your vacuum advance idea.

I recall reading that moving it to the radiator support was a way to locate it on a less hot environment.

Maybe move it to the front side of the radiator support, putting it kind of outside the oven?

To clarify, i got it snug with socket using finger strength. I did not use my 1/4" drive socket wrench this time.

Next test will be to swap gm module, as i have a spare.

I’ve tried running fuel pump several times, it makes no difference to my hot idle issue. My car starting in this condition seems to be OK. I do always hear a ping upon start, I assume that’s fuel vapor in injectors getting cleared.

I think this is true. Like I said, popping the hood at shutdown seems to cure the issue. I may experiment with a small computer CPU fan aimed at module (I do have a heat sink on top of mine), just hack something in and run it for 20 minutes after shutdown.

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FWIW, letting the car sit for 20-30 minutes heat soak DID destroy my non GM Module a few years ago. No start.

For all we know, the GM Module is almost indestructible to overheating, but does act up with excess heat. The key would be to read the output signal from it during this hot idle issue (the one it sends to ECU). But I am not an electronics expert, nor even a beginner :wink:

And by the way, the whole reason I’m revisiting this…went for a nice 3 hour drive with my local Jag club. We stopped every once in a while for 30 minutes. Every time, I had to nurse my car to a nice idle before I could rejoin. Embarrassing! :frowning:

In comparison, the Marelli XJS’s did not have that problem.

Can one take the module out of the little box Jag put it in and mount it somewhere else? It can’t be helping to be inclosed and an extra layer of metal between it and whatever it could be dissipating heat into.

I know someone here mounted it with heatsink alone on their electric fan enclosure, and had good results.

I really want to avoid the hassle of relocation if I can.

General Motors put this module inside an almost airtight distributor and called it HEI probably from 1973 to1986 in more than likely 100 million cars. I owned a few and never once did I experience any of these traits.

Admittedly I did see one fail in my buddy’s father’s Buick, but it was an easy fix.

Makes me wonder how all these problems can be the same Delco part.(just saying)

GM had their distributor tucked way in back, correct? Not sitting above intake manifold sitting on top of exhaust manifold.

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Back in the 70s-80s I worked in a GM (Oldsmobile) parts department. Those modules were fast sellers. I even remember the old part number: 1875990 ! I reckon we sold a dozen a month at least.

The most common failure mode by far was failure after absorbing a lot of heat……such as a 2-3 hour drive. After cool down, all was well in many cases. Generally, the older the module the more likelihood of failure.

Of course many many thousands of them didn’t fail, for whatever reasons… which might include never being exposed to the conditions that were most likely to induce failure!



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I have forgotten how many times I pulled a 28 foot boat on a triaxle trailer across the southern US with a 455 cu. in. Toronado with almost 100 thousand miles on it. And I don’t mean two hrs and then a break, it was almost non stop.

That car was driven hard and put away wet many many times. Also went over 80,000 miles on original plugs.

I was not then nor now a fan of GM product especially FWD., but what a workhorse it was.

Forgot to add but with totally worn electrodes and driven in middle of winter it finally failed to start and even then that huge load on the module it survived many more miles and a couple of years.

No , it was at the front. Not sure , but I think the 455 was a Buick engine. Their engines were trademark front distributor. Unlike Chevy.

Oldsmobile, Buick and Pontiac each developed their own 455. Olds was first on the market with Rocket 455 in 1968. Buick was 1970 but their engine was lighter…they bored out their 425.

I remember the 425 great engine.