Hesitation at mid rpms under load - revisited
09-06-2005, 12:44 PM
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Hesitation at mid rpms under load - revisited
I dug up an old thread and posted to it here >>https://rennlist.com/forums/924-931-944-951-968-forum/117188-stumbling-around-3000rpm.html Here are my results and findings...
I know a lot of us have experienced the mid-range rpm lean conditions, with slight hesitation and surging under part throttle load and slight burbling on closed throttle deacceleration, indicating too much air. On full throttle and most of all the rpm range, my engine runs fantastic, just this little surging and hesitation that really bugged me and I wanted to fix. One thing I noticed, was that the hesitation was not there when the engine is cold. At first I thought it might be my O2 sensor was bad, but some responded that hooking up this made symtoms worse. I installed a new one anyway and have left it hooked up through this all.
So far I have;
- Retracked my AFM wiper arms (smoothed out low rpm reving)
- New O2 sensor (old one was bad now goes into 14.7 closed loop)
- Checked ohm values on water temperature sensor on head (ok)
- New spark plugs, cleaned cap, rotor
- Wurth dielectric grease on all engine connectors
- Wurth dielectric grease on all injector connectors (by rotating the injectors seemed to change the symtoms some... hmmm)
- Carrera Cup style air filter housing and K&N drop in filter
What is happening is that as the engine gets to operating temp it get leaner and the slight surging/hesitation under load is more noticable. I thought about this and the two items that tell the DME to control fuel mixture are the water temperature sensor on the head and the O2 sensor going into closed loop. I first checked my readings on my temp sensor - everything specs out ok, it's doing exactly what it is supposed to do. So, I thought let's try this - I have an extra known good water temperature sensor and left the one in the head but just unplugged this and plugged in my spare. First I stuck it down on the head next to the installed one in the head so it took a somewhat close reading - same hesitation symptoms. Ok, this is getting me cranky. Next, I pointed it up in the air nestled next to the j-boot. This way I thought it will offer a cold start enrichment all the time.
Viola! Problem diagnosed.
I don't know how much fuel the cold start enrichment provides but, leaving a known good temp sensor exposed to ambient air is just enough to compensate for the air leaks and get rid of the lean condition. We must all have horendous intake air leaks - everywhere! The Porsche 944 engine is a masterpiece, a sensitive absolute beautiful running jewel, and must need to be put right to original condition for optimal running conditions. I guess it's time to R&R the rubber parts of the intake.
What to do next?
- Replace every intake, vacuum line and gasket
- Replace o-rings seals on throttle body
- Replace injector seals
- Replace j-boot
- Replace oil fill cap gasket
- Anything else I missed?
I'll report on this again. At least I've found the culprit and can proceed to repair.
Thompson
1985 944.1
Club Sport targa
I know a lot of us have experienced the mid-range rpm lean conditions, with slight hesitation and surging under part throttle load and slight burbling on closed throttle deacceleration, indicating too much air. On full throttle and most of all the rpm range, my engine runs fantastic, just this little surging and hesitation that really bugged me and I wanted to fix. One thing I noticed, was that the hesitation was not there when the engine is cold. At first I thought it might be my O2 sensor was bad, but some responded that hooking up this made symtoms worse. I installed a new one anyway and have left it hooked up through this all.
So far I have;
- Retracked my AFM wiper arms (smoothed out low rpm reving)
- New O2 sensor (old one was bad now goes into 14.7 closed loop)
- Checked ohm values on water temperature sensor on head (ok)
- New spark plugs, cleaned cap, rotor
- Wurth dielectric grease on all engine connectors
- Wurth dielectric grease on all injector connectors (by rotating the injectors seemed to change the symtoms some... hmmm)
- Carrera Cup style air filter housing and K&N drop in filter
What is happening is that as the engine gets to operating temp it get leaner and the slight surging/hesitation under load is more noticable. I thought about this and the two items that tell the DME to control fuel mixture are the water temperature sensor on the head and the O2 sensor going into closed loop. I first checked my readings on my temp sensor - everything specs out ok, it's doing exactly what it is supposed to do. So, I thought let's try this - I have an extra known good water temperature sensor and left the one in the head but just unplugged this and plugged in my spare. First I stuck it down on the head next to the installed one in the head so it took a somewhat close reading - same hesitation symptoms. Ok, this is getting me cranky. Next, I pointed it up in the air nestled next to the j-boot. This way I thought it will offer a cold start enrichment all the time.
Viola! Problem diagnosed.
I don't know how much fuel the cold start enrichment provides but, leaving a known good temp sensor exposed to ambient air is just enough to compensate for the air leaks and get rid of the lean condition. We must all have horendous intake air leaks - everywhere! The Porsche 944 engine is a masterpiece, a sensitive absolute beautiful running jewel, and must need to be put right to original condition for optimal running conditions. I guess it's time to R&R the rubber parts of the intake.
What to do next?
- Replace every intake, vacuum line and gasket
- Replace o-rings seals on throttle body
- Replace injector seals
- Replace j-boot
- Replace oil fill cap gasket
- Anything else I missed?
I'll report on this again. At least I've found the culprit and can proceed to repair.
Thompson
1985 944.1
Club Sport targa
09-08-2005, 07:17 AM
#3
Rennlist Member
Excellent find! And thank you for PM'ing me about it. I am confident I am due for the J-boot change and TB seals. I do have new intake gaskets and all new vac lines as of 1 year ago. I do need an oil filler gasket too. Let us know what your results are. Thank you again!
BTW: what does your Carrera Cup air filter housing look like?
BTW: what does your Carrera Cup air filter housing look like?
09-08-2005, 11:15 AM
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One plus result that came with this diagnosis finding is that more fuel and more air usually = more power. 
Honestly, since I did this, the Porsche runs like a hoot - never been faster. With my Carrera Cup style air box K&N filter mod too it really runs good up top - way better than before. Throughout the mid range rpms it feels like it's got more there. I'd like to find out exactly how much fuel percentage the temp sensor dials back as the DME gets warmer, or how much the cold starting fuel increase percentage is. It's not alot, but left cold, enough to cover up the air leaking hesitation. I know, I'll mount up a 937 hood air scoop and put the water temp sensor there instead of fixing my intake leaks! 
Thompson
1985 944.1
Club Sport targa
Honestly, since I did this, the Porsche runs like a hoot - never been faster. With my Carrera Cup style air box K&N filter mod too it really runs good up top - way better than before. Throughout the mid range rpms it feels like it's got more there. I'd like to find out exactly how much fuel percentage the temp sensor dials back as the DME gets warmer, or how much the cold starting fuel increase percentage is. It's not alot, but left cold, enough to cover up the air leaking hesitation. I know, I'll mount up a 937 hood air scoop and put the water temp sensor there instead of fixing my intake leaks! Thompson
1985 944.1
Club Sport targa
09-08-2005, 11:19 AM
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Carrera Cup style air filter mod. Take a jig saw and cut the entire stock air filter top off. You'll be left with the square frame and the screws to hold the stock filter element in place. Not pretty, but effective because it RETAINS the stock velocity stack that's tuned for the engine. I'll snap off a photo tonight.
Thompson
1985 944.1
Club Sport targa
Thompson
1985 944.1
Club Sport targa
Last edited by Der Rennwagen; 09-08-2005 at 11:38 AM.
