1987 911 3.2 misfiring when engine is cold

[proporsche]

cabmando..we agree to disagree..The official Porsche manual is here exactly for this.Any regular maintenance when is done on the 911 requires checking fuel mix and c/o adjustment...
That is how we were trained back in the 1980`s on Porsche meeting;-)

Ivan

[proporsche]

Johan glad you git it better.and running.And yes as Cabmando said those repaired hoses you should replace....
Ivan

[cabmandone]

Originally Posted by proporsche

cabmando..we agree to disagree..The official Porsche manual is here exactly for this.Any regular maintenance when is done on the 911 requires checking fuel mix and c/o adjustment...
That is how we were trained back in the 1980`s on Porsche meeting;-)

Ivan

The problem is, on a 3.2 you can mask an issue by playing with the mixture screw on the AFM. If the injectors, fuel supply system, and ignition system are up to snuff, you shouldn't need to touch that screw. I've been running my car without touching that screw since I set it 3 years ago and I have a wideband 02 with AFR gauge to monitor what it's doing. I can tell when I've left the oil cap off with sound deadening ear plugs in. I just look at the AFR. On the CIS systems I think it's more of a maintenance thing than on the 3.2 Motronic system.

[proporsche]

You might have some wideband..that is nice..me ..i do not need anything like that, i have 40 year(correction 36) of exp.working on these beasts and my years and hands ..that is all i need;-)If i would leave the oil cap off ,i can see it;-))))

Ivan
anyway all good....everybody does what they know how to do it;-))

[Johan91132]

Gues it is best to reply to this old thread, so if anyone is enough interested it is easier to get the context of my problems.

Well anyway i was quite busy last fall and didn't have time to/remember to update this thread. But indeed my 3.2 started to work after taking it to a 911 mechanic. He ultrasonic cleaned the injectors and changed the plugs. All was good for about 200 miles, then the car started misfiring again.

Nothing helped. Tried to clean the distributor cap, change a new rotor, change plugs but no change to the misfire. Only thing that is yet to be changed from the ignition system is the distributor itself. The car went to winter sleep after approx. 2000usd spent on workshop and parts.

Took it to the same mechanic at the end of april, and he yet again cleaned the injectors and changed the plugs. Car idled a bit rough and it cost 400usd so quite a headache but thought it was good enough and pulled strongly.

Well after about 200 miles it started to misfire again. Ordered new injectors seals, took off the injectors, cleaned them and after cleaning the intake holes where they touch and installing them, the car ran better than ever. Noticed about 30hp power increase and insane throttle response. Car ran fine for about 500 miles, with some slight hesitation/misfires with two startups that went away.

After 700 miles with new injectors seals the car started to misfire like crazy. Took off the plugs and one was very oily and carbon fouled, and other two were also quite bad (these plugs are the victims of the fall 2020 misfires, dunno if the mechanic cleaned them when working on the car couple months back) Changed the plugs, car misfired for the first 5 miles but then worked like new for the next 200 miles. Then it started to misfire extremely badly and misfired for about 80 miles of highway driving. After a half an hour stop with warmed up engine the car ran again good as new.

That was a summer holiday trip, so i really did not have an option to not drive it while it was misfiring. Today i started it again after 4 days of not driving and got constant misfires no matter if the engine was hot or cold.

Really loosing all hope here. Closer to 3000usd spent from my student budget to this unreliable joke of a car. My family has had the car for 18 years but i've been looking for some more sensible options lately (chipped 114i). Friends "never serviced" 400k miles driven volvo 760 is a lot more reliable even at sub-zero temperatures.

What options do i even have? I've lost all hope and trust to every mechanic and can't afford too big of an overhaul (rebuild) at the moment. Engine should be in good condition with 75000 miles and 12 bars of pressure in every cylinder. Car works like a dream when it works. But the misfires make 0 sense and come so randomly, that i'm falling to depression soon because of this car. It takes all my money and freetime and gives close to nothing back.

Any advice greatly appreciated.

[Johan91132]

Proof of the car working as intended (video shot day before i took of the oily carbon fouled plugs after the car started to misfire badly, gues one cylinder stopped working completely): https://youtu.be/iaF25CREkOM

Also attached a picture of the plugs i took of.

[cabmandone]

I went back over the posts and don't see any mention of the cylinder head temp sensor being tested. You might have a look. Your plugs say it's running ridiculously rich. A CHT failure can make it run rich.

[mysocal911]

Originally Posted by cabmando

I went back over the posts and don't see any mention of the cylinder head temp sensor being tested. You might have a look. Your plugs say it's running ridiculously rich. A CHT failure can make it run rich.

Typically the CHT sensor fails open, resulting in black smoke out the tailpipe and a dying engine when the engine is warm.

[mysocal911]

Originally Posted by cabmando

The problem is, on a 3.2 you can mask an issue by playing with the mixture screw on the AFM.

Remember, the AFM adjustment screw only affects the mixture at idle with the O2 sensor disconnected.

[stlrj]

I have found that the most common problem the 3.2 engine has is the fact that they came with Beru ignition cables which I consider to be highly unreliable and very much prone to miss fire. Try turning the lights off at night in your garage with the engine running and observe the light show and you will see what I mean. Meanwhile, spray some windex on the cables and watch the sparks fly while your engine miss fires and possibly dies.

[cabmandone]

Originally Posted by mysocal911

Remember, the AFM adjustment screw only affects the mixture at idle with the O2 sensor disconnected.

When the engine is warm... right? If I'm not mistaken, on initial cold start the o2 sensor is ignored.

[Johan91132]

Originally Posted by cabmando

I went back over the posts and don't see any mention of the cylinder head temp sensor being tested. You might have a look. Your plugs say it's running ridiculously rich. A CHT failure can make it run rich.

This is one of the only things that comes to my mind before actualy opening the engine. Might have to take a look. Haven't seen any black smoke from the tailpipe tho.

[Johan91132]

Originally Posted by stlrj

I have found that the most common problem the 3.2 engine has is the fact that they came with Beru ignition cables which I consider to be highly unreliable and very much prone to miss fire. Try turning the lights off at night in your garage with the engine running and observe the light show and you will see what I mean. Meanwhile, spray some windex on the cables and watch the sparks fly while your engine miss fires and possibly dies.

The ignition cables were original when i changed them to new Beru ignition cables last year. I doubt there will be a light show but might have to take a look.

[cabmandone]

How many miles do you have on the new plugs? Have you looked at them to see how the engine is burning? It seems odd that you have 4 fouled and two that are okay to maybe a little lean. One really easy check that I do to check balance is take an IR thermometer and check the exhaust ports going into the heat exchangers. It sounds to me, unless I missed something, like you're okay at idle but it starts to run rougher as the rpm's pick up. I think that's another indication of the CHT.

You could start by testing the resistance of the CHT. See post #2 in this thread
Verify my CHT sensor test results

[mysocal911]

Originally Posted by cabmando

When the engine is warm... right? If I'm not mistaken, on initial cold start the o2 sensor is ignored.

Again remember, the 911 3.2 DME ECM controls an idle valve which bypasses the throttle butterfly minimizing the effect of the mixture screw.
Furthermore, the mixture is fairly rich anyway when the engine is cold.

Bottom line: Set the mixture screw three full turns out and forget it!

[mysocal911]

Originally Posted by cabmando

You could start by testing the resistance of the CHT. See post #2 in this thread
Verify my CHT sensor test results

Let's not get overly complicated. When the engine is warm, just jumper the CHT with a paper clip to eliminate it as a problem source.

Remember, once the CHT gets below 500-700 ohms, it has basically no effect!

[manbridge 74]

In addition to the plug wires I’d checked the injectors again. New ones I see, but where did you source them? Had a customer car recently, injectors were already replaced but it didn’t last long. I put in Porsche injectors, car hasn’t come back.

[cabmandone]

Nothing "overly complicated" about what I suggested. And since the engine isn't right and is backfiring, I'm not sure running it until it's fully warmed up to do the paperclip test is a solid plan. You might consider going back and reading his OP, then going forward. Then... rather than nitpicking other posts... suggest something that might actually help the OP??

[cabmandone]

Originally Posted by mysocal911

Again remember, the 911 3.2 DME ECM controls an idle valve which bypasses the throttle butterfly minimizing the effect of the mixture screw.
Furthermore, the mixture is fairly rich anyway when the engine is cold.

Bottom line: Set the mixture screw three full turns out and forget it!

Engine warm or cold? Doesn't matter? Vaccuum leaks... no matter! Just 3 turns out and call it a day!

[stlrj]

Originally Posted by Johan91132

The ignition cables were original when i changed them to new Beru ignition cables last year. I doubt there will be a light show but might have to take a look.

Don't take my word for it. Make your own observation. Ever wonder why cars today no longer use ignition cables? Think about it...even new Beru cables are junk!

Why do all cars run coil on plug ignition and have eliminated ignition cables entirely?