1990 964 C4 - motor shuts down when decelerating

[Bedo]

I've noticed this quirk from the day I bought and drove the car back to Minneapolis from Boston. Whenever I'm coasting down longer grades (whether I'm going 35 or 75), the motor will shut down (if I don't disengage the clutch). This tends to be an intermittent problem so of course the day I finally bring it into the dealer for repair (it's been getting worse), it disappears. Reading the posts here (since it's now in the shop), I figure I may as well have the dealer pull and inspect; the distributor for cracks, the distributor internal drive belt and the condition of the coils. From what I've read/understand it could be a matter of replacing the coils. Any thoughts would be appreciated.

[efhughes3]

Isn't there an idle control switch? There was on my '84. If it is inoperative, the closed throttle will choke off the engine. I've not gotten far enough in the later models yet to know if there is similar on 964's and 993's, but idle think there would be some form of idle circuit.

[Bedo]

The car starts and idles great around 800rpm, whether it's below zero or there's 90% humidity. This quirky problem is only related to there being a drag on the motor when decelerating.

[adeak1]

Had same problem,

I changed the ignition coils(2) in the engine compartment. It work for Me

Andrew

[RetiredRider]

Same problem I had. In addition to coils I changed ICUs, plug wires, plugs, dist caps, rotors. You may have to do the same. You can check plug wires by looking at the engine at night. If you see sparks you will need new wires.

[Bubbling well]

I had a same kind of symptom when I was coasting downhills with my right foot off the gas pedal. Found there was too much oil in the tank. Drained some oil and cleaned the Idle Control Valve. After that car runs just fine.

[Stian911]

Originally Posted by Bubbling well

I had a same kind of symptom when I was coasting downhills with my right foot off the gas pedal. Found there was too much oil in the tank. Drained some oil and cleaned the Idle Control Valve. After that car runs just fine.

Where is the "Idle Control Valve"? I have the same kind of problem

[bazar01]

The idle control valve is the silver looking cylinder with two attached hoses on top of the intake manifold.

Just wondering:

Where is the combustion air coming from when coasting in gear or engine braking , knowing the throttle is closed with the foot off the gas pedal at >35 mph or above idle speed to support combustion and keep the engine running? Is the throttle being forced open by the engine displacement or the idle control valve opening is enough to open the airflow meter barn door to provide the fuel map?
In short, what keeps the engine running during engine braking with the foot off the gas pedal?
Sorry if this is a stupid question.

[phheitlinger]

I would replace the battery before I did anything more expense. When my battery had low voltage (going bad) I would experience the same issue. The ECU will get real squirrely at lower than normal voltage.

[canuck964]

+1 on the battery replacement.

I had the same issue.

I purchased a "Optima Red Top" battery three years ago. As I only drive it during the summer I used a regular battery tender to keep it topped up in the winter. It turns out that the gell type of batteries takes a specific type of battery tenders. After the second winter, it failed to start up the car and I just thought maybe someone had unplugged the battery tender during the winter. So I recharged the battery and all seemed well.

Except I started to have stalling issues all summer. It turns out the battery had started to fail and because it wasn't accepting the charge properly, the alternator was putting a load on the engine all the time.

I bought a new battery and now all is well.

[Stian911]

I LOVE YOU GUYS! I had the same problem with my 964. The engine would run erratic and suddenly stop when slowing down. Today, after reading this forum, I bought a new battery and guess what?? Problem solved! Car runs perfect at idle and does not stop.

[Stian911]

Looks like I might have jumped the conclusion there. Just came back from a longer drive. The car acts pretty much like before the battery change. What would be the next thing to try? Clean the ICV?

Thanks!

[Stian911]

Now I have been om a 2 day trip in Norway with my 964. Before I left home I drained some oil out of the engine, and now the engine is running perfect! No stalls and at idle the rpm is steady! Love it!

[canuck964]

Stian911:

I suspect that there are multiple causes for the stalling and that a combination of solutions is necessary.

Really feels good when it is finally solved.

It looks like a lot of the road in Norway are similiar to what we have here on the coast of British Columbia.

[greg1990964]

Check the hose connecting to the idle control valve. When my car gets hot the hose is collapses almost flat and chokes out the motor and makes it stall. You can see the hose expand and collapse by revving it up by hand in the engine bay. ordering new hose now for mine. 22 year old hose became very soft and pliable.

[RW964]

Originally Posted by greg1990964

Check the hose connecting to the idle control valve. When my car gets hot the hose is collapses almost flat and chokes out the motor and makes it stall. You can see the hose expand and collapse by revving it up by hand in the engine bay. ordering new hose now for mine. 22 year old hose became very soft and pliable.

Hey whist you are in there check your ISV Idle Stablization Valve. You can clean it with carb cleaner you should hear the valve moving inside when shaking it . I am going to take a close look at my hoses after reading these post just to see what I have ...I have no problems but want to stay ahead of any. Just got back from B C had a good time took a cruz up to Alaska and stopped in B.C. a couple of times just a great place to be .....