Car runs poorly after being left to cool for 20 minutes

I’ve recently restored a 1988 Alfa Romeo 75 3.0 V6. Previous work done in this thread:

The car starts and runs perfectly now, however the only issue comes when the car has been turned off for about 10/20 minutes after a drive. When starting back up the idle is rough and it stalls so the accelerator must be pressed to keep it going. It usually gets running back to normal after a few minutes.

Any ideas?

  1. bad Idle air control valve
  2. clogged air filter
  3. clogged fuel filter

Also might be vapor lock caused by an injector that leaks down while the engine is shut off. Checking the fuel pressure would tell you if this is the problem.

We can throw parts at it all day. What needs to be done is an accurate assessment of engine sensors while it’s failing. You seem to be experiencing heat soak of some sort. Your first thread has a lot of signs of fuel pump failure, did you address that?

Heat soak was my first thought too.
And I agree that it needs to be diagnosed by a competent diagnostician at a good shop. If perhaps this is carbureted (some carbs were still around in '88) you might seek out an old time mechanic knowledgeable in carbs.

And be prepared for him to find more than one contributing cause. Fuel system problems and ignition system problems can exist together and make sorting the causes out difficult.

The fuel is fuel injection, and doesn’t have a diagnostics system. What exactly is heat soak? The car runs fine for hours even in slow traffic so when it’s very hot. The problem only occurs when the car has been cooling for a while

I’ll investigate into the fuel pump, although it is brand new. It could be another blocked filter, since the tank was corroded. Just strange that the car runs fine until its left to cool for a while

When you park your car, you’re effectively shutting off the airflow that helps keep everything cool. It’s not unusual for underhood temperatures to climb for a little while after you shut off; that’s why some manufacturers run the electric fans even with the key off. What you’re likely experiencing is one of your vital engine management sensors is getting so hot that it starts to malfunction, and returns to normal once it cools a bit from driving. Potentiometers like throttle position sensors can do this, MAF’s can do this, coolant temp sensors, intake air temp sensors. You’re going to want to dig up the shop manual and find the specs for what they should be, and test while failing. Eventually you’ll find one that isn’t right.

Heat soak is when the car’s shut off and the engine heat ‘soaks’ all the equipment (and gasoline) under the hood. In FI engines the fuel system should be held at high pressure even when shut off, eliminating fuel boiling (‘vapor lock’) in the fuel system. Cars with carbs operate at much lower fuel pressure, and can have problems. If one of your fuel injectors, or a valve in the fuel pump, is leaking, you lose the high pressure and the gas boils, causing problems like you’re seeing.

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Thanks for the replies. Heat soak does sound like a very likely cause. I tried leaving the bonnet open when i left the car, and when i came to start it again it ran a lot better

That sort of confirms it.
Understand that the temperature in the engine compartment rises after you shut the engine off, and that can affect failing electrical components that run fine when running. When the engine is running, air flowing through the engine compartment and coolant flowing through the engine keep the heat from building too high, but when you shut the engine off all the engine’s internal heat (it’s a whole lot hotter around the cylinders than it is on the engine’s surface) radiates out the sides of the engine and exhaust manifold and cause underhood temperatures to spike.

Components with coils in them are the most susceptible to the problem. If your '88 Alpha has either a coil or a coil pack, that’s the first place I’d look. You’ll want to check for ignition pulses with the coil(pack) heat saturated. My guess is that they’ll be weak at best. You may need a friend with a scope to do this.

If it has a pickup coil (AKA signal generator, etc.) in the distributor, it can fail intermittently when hot. Without a steady signal to the rest of the ignition system, there will be uneven spark.

My 1979 Toyota truck had one of these. The truck would sometimes run evenly or quit in the city on a hot summer day. That fall it simply would not fire up. Luckily it was in the parking lot of an auto parts and service store, and they were able to diagnose it quickly and get it repaired the next day.

You had a '79 Toyota pickup too? Cool. I loved mine.

These igniters became common in all cars, phasing points and condensers completely out. And they can, indeed, become heat sensitive. As well as moisture sensitive.

But the one I was thinking about was the ignition coil. The “signal generator” only replaces the points system to properly time and direct the spark. The ignition coil(s) create(s) the high voltage spike necessary to jump the gaps in the sparkplugs.

The ignition coil creates a complete circuit for the 12VDC to encircle the coil’s core and create a magnetic field. The “signal generator” allows the circuit to break at the appropriate time for each individual sparkplug to receive a high voltage spike created by the magnetic field collapsing into the coil’s iron core. The circuit is then closed again and the coil’s magnetic field allowed to build again in preparation for the next sparkplug’s demand for a voltage spike.