1987 911 3.2 misfiring when engine is cold

[Johan91132]

The intake manifold bolts were indeed quite loose, the worst was at about 10nm torque i would say. Had to tighten them by hand because lacking some parts to my torque wrench, but got them to about 20-30nm by hand.

Wen't for a test drive after all the stuff mentioned above and the result was the same as always with misfiring from startup which ended after 10 minutes of driving and the engine heating up. Today was a warm and dry day so the misfiring has evolved to the next level it seems. One thing i noted today was that after a hard 2nd gear pull i get a loud backfire when changing to 3rd. The car was also backfiring when downshifting and such. Not too much but it was noticeable when driving windows open.

After that i was fed up and decided to try to alter the fuel to air ratio manually from the dme. Changed the fuel quality switch from the motronic to position 3, which lowers the amount of fuel coming to the system by 3.9% it seems: https://www.911chips.com/fuelsys.html

After that i got the first cold start in a month without misfires! The car run very nicely and didn't backfire noticeably after hard pulls, some backfiring still sometimes i.e when rolling downhill while engine braking with about 3-4k rpm.

This probably isn't the best long term solution, but proves that the engines air to fuel mixture is off by quite a bit. Could it be a faulty or dirty air flow sensor? I'm seriously running out of ideas, could the valve guides be still the cause?

[proporsche]

hi,when was the last time you did see the C/O fuel mix being set?And what is is it set up to?
Ivan

[Johan91132]

Originally Posted by proporsche

hi,when was the last time you did see the C/O fuel mix being set?And what is is it set up to?
Ivan

I'm not even sure what that is, so it hasn't been set probably never.

[proporsche]

I would start with that ,to do that you have to go to pro Porsche shop or anybody who has a gas analyzer to set your fuel mix....this is the basics, before you start trouble shooting...
If that has not been done on your 911 you might have to remove AFM and remove the factory plug.Under there is a adjustment screw to correct the fuel mix...
look here

Also to set the correct fuel mix you have to measure before catalyst-if you have one on your 911...and oxygen sensor disconnected..which you can actually leave disconnected -i do not know how is in Finland with smog tests every 2 years?

[proporsche]

to set the idle...

[Johan91132]

Originally Posted by proporsche

I would start with that ,to do that you have to go to pro Porsche shop or anybody who has a gas analyzer to set your fuel mix....this is the basics, before you start trouble shooting...
If that has not been done on your 911 you might have to remove AFM and remove the factory plug.Under there is a adjustment screw to correct the fuel mix...
look here

Also to set the correct fuel mix you have to measure before catalyst-if you have one on your 911...and oxygen sensor disconnected..which you can actually leave disconnected -i do not know how is in Finland with smog tests every 2 years?

Oh ok i had never heard about this. Has it something to do with carbonmonoxide levels in the exhaust gases or why is it called CO adjustment? Why does it have to be adjusted now? Has the engine wear reached a point where it is necessary to get proper fuel to air ratio? Why do i have to drill the AFM? Is this an unofficial hack or is it in the factory maintenance manual? Sorry for so many questions but i am absolutely clueless and you seem to be an expert regarding 911 3.2's, absolutely golden information so far!

And my 911 does not have a catalyst or oxygen sensor so that makes the procedure a bit easier.

By the way, today was very rainy day and the misfires came back. They weren't as bad as before and stopped at about 50⁰C oil temp instead of 70⁰C as was the case before the motronic adjustment.

[Nick Triesch]

Loose intake bolts will cause rough idle And a mis that will get worse over time. I had the problem. A very experienced a Porsche mechanic in San Diego told me to tighten them up and it did the trick. After several years on a 3.2 the bolts became loose. Common.

[proporsche]

you are so funny............ "unofficial hack"..dude those are Porsche factory pages.It is basic maintenance(usually once in a year or 2 years) to check the C/O level and adjust ....i`d bet your needs to be adjusted...

for you miss when is raining or wet ...start with you spark plugs connector..when dry weather Use spray bottle and when the engine is running spray on each cylinder at a time.trying to duplicate the miss...

[Johan91132]

Originally Posted by Nick Triesch

Loose intake bolts will cause rough idle And a mis that will get worse over time. I had the problem. A very experienced a Porsche mechanic in San Diego told me to tighten them up and it did the trick. After several years on a 3.2 the bolts became loose. Common.

Makes sense, but in my case it didn't do a difference even though the bolts were very loose.

Originally Posted by proporsche

you are so funny............ "unofficial hack"..dude those are Porsche factory pages.It is basic maintenance(usually once in a year or 2 years) to check the C/O level and adjust ....i`d bet your needs to be adjusted...

for you miss when is raining or wet ...start with you spark plugs connector..when dry weather Use spray bottle and when the engine is running spray on each cylinder at a time.trying to duplicate the miss...

I'm so extremely busy with work at the moment that i didn't have time to properly read the pictures until now. Realized that the instructions don't include drilling a hole to the metal bits of the AFM, just removing the plastic plug haha. Because of that i thought it was somekind of hack. The procedure seems simple enough but i have to take the car to a local porsche mechanic because i don't have the gas test devices.

Well yesterday was as wet as it can get when i left work before midnight and the car ran perfectly so now i'm even more bafled...Also i have diagnosed that the misfire goes away when i "wake up" the engine by accelerating hard over 4k rpm. It would probably go away instantly after start if i'd accelerate, but don't wanna stress the cold engine. And the worst misfire happens in cylinders 6 and 2 (the old plugs in those weren't only carbon fouled, but also a bit oily).

[proporsche]

The plug you have to remove is Alu.plug not plastic.it is held in with a C clip on it`s end.The best how i do it is ,drill 2 little hole about 4mm between then use needle nose pliers and pull it out.Pretty safe stuff no worries.Once you have it check the fuel mix and adjust it,as it is on the manual instructions.
Still,if you want to duplicate the wet condition as i have suggested get a spray bottle with water and while the engine is running spray on each cyl..you see any miss right away...

Ivan

[Johan91132]

Originally Posted by proporsche

The plug you have to remove is Alu.plug not plastic.it is held in with a C clip on it`s end.The best how i do it is ,drill 2 little hole about 4mm between then use needle nose pliers and pull it out.Pretty safe stuff no worries.Once you have it check the fuel mix and adjust it,as it is on the manual instructions.
Still,if you want to duplicate the wet condition as i have suggested get a spray bottle with water and while the engine is running spray on each cyl..you see any miss right away...

Ivan

Oh ok still a plug though, seems simple enough. Maybe i should clean the AFM and throttle body while i'm at it? Seems like a good idea, and spray the water from under the car or..?

[proporsche]

no spray the water on each cylinder on top to try to duplicate the wet weather.Not so much only little moist will find the crack or something in your system..the plug looks like this..when removed..bottom of it

[gomezoneill]

I think you may have an induction leak of some kind. Maybe some of the intake gaskets fell apart because of the intake being loose. You might want to pull the manifold and check the seal of said gaskets. You could try using a vacuum hose up to your ear and listen for a sucking sound around the intake at the heads.

[firstclassof86]

i had a similar issue with fouled plugs that got progressively worse. Long story, short. It was a bad fuel damper. Simple test is to disconnect the vacuum line located on top of the damper and see if it has any fuel or smells like fuel (note: it should not have any fuel ). if it does, then you have a tear in the diaphram inside the damper and it is allowing unmetered fuel into your intake manifold. The DME is now trying to compensate for the extra fuel and your car is running bad.

[Johan91132]

Originally Posted by proporsche

no spray the water on each cylinder on top to try to duplicate the wet weather.Not so much only little moist will find the crack or something in your system..the plug looks like this..when removed..bottom of it

Ok will test that the next time i get the engine running. Seems like there wasn't a plug in this particular afm(attached picture). Is the hole with the hex key bolt the right one? Also it seems to be impossible to open the afm for inspection, removed the sealant around the black plastic lid and tried to pry it open with a screwdriver with no luck. Any tips?

Originally Posted by gomezoneill

I think you may have an induction leak of some kind. Maybe some of the intake gaskets fell apart because of the intake being loose. You might want to pull the manifold and check the seal of said gaskets. You could try using a vacuum hose up to your ear and listen for a sucking sound around the intake at the heads.

Sounds like a good idea, but at the moment i have no time to perform such large scale disassembly, maybe later in the fall. The hose trick seems good though, will try it.

Originally Posted by firstclassof86

i had a similar issue with fouled plugs that got progressively worse. Long story, short. It was a bad fuel damper. Simple test is to disconnect the vacuum line located on top of the damper and see if it has any fuel or smells like fuel (note: it should not have any fuel ). if it does, then you have a tear in the diaphram inside the damper and it is allowing unmetered fuel into your intake manifold. The DME is now trying to compensate for the extra fuel and your car is running bad.

Well this is a very good advice, the damper is the small metal box far right at the engine bay? Noticed it while disassembling the airbox and afm.

[proporsche]

Johan, do yourself a favor..do not start taking things apart until you know where the problem is...
friendly advice....;-)

Ivan

[Johan91132]

Originally Posted by proporsche

Johan, do yourself a favor..do not start taking things apart until you know where the problem is...
friendly advice....;-)

Ivan

That could be a good idea haha, but i like to get to know the car in the process + figured i'd save quite a lot of money by removing the plug myself before taking the car to a 90€/hour porsche repair shop. In a hindsight i couldve just felt the plug being missing or something.

After removing the afm & airbox assembly i noticed a hose that was a bit damaged. The hose led to the throttle body, no idea what it does. I put some high temp silicone glue to the broken hose as a temporary fix to see if it affects anything. Picture shows the damaged hose.

[proporsche]

well you do as you do and want .i have my 911 for 24 years and i had never ever need to take off the throttle body;-))))let`s hope you got new o ring for it;-)

Ivan

[Johan91132]

Originally Posted by proporsche

well you do as you do and want .i have my 911 for 24 years and i had never ever need to take off the throttle body;-))))let`s hope you got new o ring for it;-)

Ivan

Wow that's my exact age haha, i've been around my 911 for 17 years but inherited it only 7 years ago when my father passed away.

Anyway i think i got the car working, didn't want to post this too soon, but after testing the car for 3 days the misfiring is indeed gone. Not sure did the cleaning of throttle body (which i didn't get a new rubber o ring by the way, the old one seemed ok though), AFM or IACV fix it.

Most probably it was the temporary high temp silicone sealant fix i did to the broken hose that goes to the throttle body. After that i had to adjust the motronic fuel quality switch back to position 0 from position 3 because the car was running lean and missfiring very badly. After the motronic adjustment 0 misfires, bit of backfiring after hard pulls though.

The vacuum line from the fuel pressure damper to the fuel pressure regulator was also disconnected from the fuel pressure regulator side (left side of the engine compartment) which could have maybe affected the engine? Noticed it while inspecting the vacuum line for fuel deposits which it didn't luckily have.

Thanks for all the feedback and help! Hopefully my joy isn't too premature. Will still get the CO mix adjusted shortly because it has been neglected for so long.

[cabmandone]

I wouldn't touch the mixture screw until you're sure your vacuum lines and everything else are up to snuff. Hoses that look cracked should be replaced. Switches should be checked for proper operation. IACV should be cleaned. Manifold boots checked for cracks. What I'm getting at is, make sure things are right before messing with that screw.

I'm going to disagree with proporsche on setting the base idle mixture being yearly maintenance. If your hoses are in good shape, your fuel pressure is right, your injectors are clean and your AFM is working properly, there shouldn't be a need to mess with that screw which is why it's capped in the first place. People pull the cap to mess with the mixture typically to overcome another issue with the car.