Engine won't rev above 1500 rpm

[JTP]

Hello everyone, i have a 1987 944S. Searched for something similar to my issue but came up empty.

First a little history, my car has been using oil at an unusual rate, a quart in 300-400 miles. Compression and leak down tests were good, borescoped cylinders and all looked good. I also have a hesitation issue that may be related to the oil consumotion.

I took off the intake manifold to replace AOS seals (siliconed the lid of the AOS also) and checked all hoses under manifold. Inside of intake manifold was oily and saw a small pool of oil on top of the valves. While I was in there I replaced the DME temp sensor. After reassembling everything it starts up, though a little rough, and idles ok. Once I get on the gas it begins to hesitate and as soon as it hits 1500 rpm it dies like it hit a limiter. It stumbles, wants to stall, then goes back to idle. I've checked and rechecked everything I touched and I can't see what is wrong. Only thing I can think of is maybe the TPS or AFM got damaged while removing the intake.

Any ideas before I have it towed to my 944 specialist?

[GGX2]

My 944 had a somewhat similar issue where it would die when it got up to 1000rpms... took me a while to figure it was the factory alarm causing it. Your problem is likely something else, but if you have an alarm you can try to bypass it. Good luck!

-George

[gtroth]

Mine behaved a little bit like that when the car was warm but the weather very cold. Chased that for a couple of years and solved it with a new AFM, so you might want to borrow a known good one to test.
But I don't burn oil (well, not in the 944S anyway :-). Might or might not be related (helpful, I know).
I'd look for vacuum leaks, clean the grounds, measure the fuel pressure, leakdown, and delivery, put a scope on the ref/speed sensor, get the injectors witch huntered, and of course try to figure out where your oil is going.

Good luck!

[Gawernator]

It's not a leak..?

[VirginiaF1]

JTP, Didn't you read your TPS memo?
(Another gem from Office Space)

Throttle Position Sensor failure maybe?

[Gawernator]

I'm watching Office Space, I swear... the Nyquil is too powerful

[curtisr]

The September 2012 issue of 911 & Porsche World magazine fields the same question (page 110).

The solution? The expert suggests that--thanks to age--the electrical contacts in the AFM can become worn, dirty or tarnished. All of this results in the loss of a 'good, clear signal for the digital side of the system'. In addition 'sometimes the shaft's support bearings can wear, too, and the resulting slight misalignment of the flap will cause it to bind against the inside of the housing. A thorough rinse with electrical switch cleaner might do the trick or a replacement is called for.' For a final thought, he adds: 'don't overlook the possibility of a poor electrical connection between the AFM and its multi-pin plug and socket. You might also want to have a look at the micro-switch attached to the underside of the throttle body that tells the electronics when the throttle plate is fully closed. Again that can become worn or loose.'

Either that or you have faulty fuel pressure regulators so check the fuel pressure.

[rhett]

Might not be connected, but always worth checking is the ignition module mounted on the wheel well just under the header tank. The connections on that can get cruddy and cause the kind of symptoms you are experiencing...I know, 'cos I had them! A good clean and WD40 to seal and lubricate might help...but good PM anyway if its not the problem.

[JTP]

Thanks for the suggestions. I will check the connections for the AFM, TPS, and the ignition module.
I did suspect the AFM before and while the engine was running I slightly pulled on the connection and felt it disengage then it began to stall until I quickly reattached it.
Also, I didn't mention I cleaned the throttle body with throttle body cleaner, if the cleaner got into the TPS would it cause a failure?

[Gawernator]

Hmm the TPS should be sealed up. I've never had that issue when cleaning the throttle body.

[JTP]

Checked the AFM, voltage output was good but the thin film was grooved so I cleaned the contacts and moved the slider to a new position off the grooves. Checked the TPS resistance at the DME connector and at zero throttle it measured anywhere from 17-49 Ohms between pin 52 and 24. Then as I moved the throttle there was no change in resistance. Moving the mutimeter to pins 53 and 24, resistance was at infinity with zero throttle and went to about 2-3 Ohms at WOT, did not go to zero. Also there is no 'click' when the throttle is moved off zero throttle. Looks like I need a new TPS.

[VirginiaF1]

Originally Posted by JTP

Checked the AFM, voltage output was good but the thin film was grooved so I cleaned the contacts and moved the slider to a new position off the grooves. Checked the TPS resistance at the DME connector and at zero throttle it measured anywhere from 17-49 Ohms between pin 52 and 24. Then as I moved the throttle there was no change in resistance. Moving the mutimeter to pins 53 and 24, resistance was at infinity with zero throttle and went to about 2-3 Ohms at WOT, did not go to zero. Also there is no 'click' when the throttle is moved off zero throttle. Looks like I need a new TPS.

Sorry for you issue mate...
But so gratified that after 2 years of reading this excellent forum,
I've been taught enough to help correctly diagnose an issue FINALLY.
GLWR

[Gawernator]

Very easy to replace

[JTP]

Should have done it when I had the intake off. I didn't want to take it off again but oh well. I'm going to see if I can take the TB off without taking the intake off first but it's worth a try.

[Gawernator]

Oh yes man. Save yourself some trouble. Just use a socket wrench with the (m6 I think) hex socket and remove the TB bolts. And the throttle cable on top. It comes off easily (might need extensions and I think I used one of those extensions that has a bendy joint thing)

[gtroth]

Originally Posted by Gawernator

Oh yes man. Save yourself some trouble. Just use a socket wrench with the (m6 I think) hex socket and remove the TB bolts. And the throttle cable on top. It comes off easily (might need extensions and I think I used one of those extensions that has a bendy joint thing)

No, not worth it on the S IMO. Pull the manifold, J-tube and throttle body in one big assembly.

[JTP]

Quote de Gawernator

Oh yes man. Save yourself some trouble. Just use a socket wrench with the (m6 I think) hex socket and remove the TB bolts. And the throttle cable on top. It comes off easily (might need extensions and I think I used one of those extensions that has a bendy joint thing)

No, not worth it on the S IMO. Pull the manifold, J-tube and throttle body in one big assembly.

Yeah took a look again and it is way tight in there. This will have to wait til next weekend.

[Gawernator]

Oooooo.. missed the S. Different design, sorry, I was thinking 2.5L NA

[JTP]

Fast forward 9 years, life events forced this project to be set aside. I didn't do anything with this car for over 9 years except to move it from house to house a few times. I haven't even logged into Pelican forum since 2013!

A month ago I decided to start work on this and finally got it running. The reference sensor ended up being the culprit, now she revs past 1,500 rpm but there is still a slight hesitation. I have come to the conclusion that the valve stem seals are shot. I have good cylinder compression but there is a lot of oil being burned, when I remove the intake manifold there is a lot of oil in the intake port with a lot dripping down onto the intake gasket. I have yet to remove the valve cover off to verify their condition but I have a strong feeling I have to replace the seals, most likely a complete head rebuild rather than replace the stem seals in place.

[Monkey Wrench]

I had similar issues after reassembling a Volvo with the flex plate on the wrong bolt. I had it turned so my car thought TDC was in a different position.
new valve seals sound a whole lot cheaper and easier than pulling the head. if you do you may be able to have the guides knurled?

I put new guides in my Volvo 122 and then took it to a decent head shop for a 3 angle grind and stainless steel seats. They knurled the new guides to make them retain oil better. Its also a way of repairing worn guides as the knurling makes the ID smaller, then they can ream them ot fit the stem.

I have not been this deep into a Porsche engine so I can't say much that is Porsche specific.

Ive seen where people go rebuilding heads then get more blow by with better compression. Hard to know where to stop. You might check compression with a tester before you go digging in deep ? It's good to have that reference anyway.

I've heard said you can do the compression check then do it again with a bit of oil in the cylinders. the oil can temporarily seal the rings and reduce blow by, but it wont affect the way the valves seal. The theory being that if you see an improvement then you might have some blow by issues.


I was fixing up an old engine in a car..It was a beater that had burned up valves. I found one valve with a larger stem and put the head on my drill press and just used the closest size bit I had. LOL... it's didn't smoke much, it just had this grey mist of oil following it. It ran after that but it took a quart of oil to get to work and another to get home. No one tailgated me ;-)

I dont know if it works with a Porsche any differently but I remember that I knocked the guts and electrode out of a spark plug then stuck a tubeless tire valve inside that. It's a handy tool, as you can compress the air in the cylinder and listen for leaks and learn something about it. also seem to remember using that to help hold the valves up and closed when working with the valve seals, keepers etc.

I was looking in a porsche manual and in there it seemed to specify "oil usage" and said something like a quart to 500 K
It made me scratch my head a bit. isn't that pretty excessive? maybe I interpreted it incorrectly.

Id think about doing the leak-down test and compression test, if not more it's interesting to see where it was before you opened the whole can of worms up. you could also find bad things that help you decide to open it up. maybe your mechanic has a camera that could look for score marks in the bore.

I think if it were me , if i didn't see compression issues , I'd try just the seals first. If you get a gasket set you might end up with a second set of them to keep as spares if you do go down the road of removing the head. if you do the head do you do the rings? or stop there? oversize pistons? new oil pump? right now I do think you are on the top edge of a very slippery (but fun) slope ;-)