Hesitations between 2k and 3.5k RPM Help :(

manu4jc · 08-24-2007, 11:41 AM

Hi Everyone,
I purchased my first Porsche a few weeks ago. It is a Guards Red 1986 944na. I got it cheap because the owner was in need of money and he was worried about the oil in the coolant tank. I purchased it for $1600 it has 103K miles on it and the interior is flawless.

I ordered the oil gasket kit for the oil cooler. But a new problem started creeping up on me this week.

For some reason sometimes when I start the car I turn the key and the engine doesn't crank...the lights come on but no crank. I turn the key back in the off position and then turn it back on and the car starts fine... Im guessing the starter is about to go?

But most of all... The car started hesitating ...real bad between 2000 and 3500 rpm. As in if I am going up a hill and it does it the car literally starts to feel like its stalling. There are no funny noises and once its at like 4k its fine. Also the problem seems more evident when I first drive the car...after 10min of driving the hesitations and sluggish acceleration are not as harsh. Oh another thing... the car has a little rough idle as in the car vibrates when it is idling...if i touch the gas the car has a normal feel no vibrations no resonance etc.

Any Ideas... Im guessing bad fuel delivery (clogged fuel filter?)

Thanks in advance,
Manu

bholmes · bholmes's Garage

I'd make sure your grounds were good first off. That can cause starting problems that may also manifest in ignition problems. Once that's OK the fuel filter sounds like a good idea to me. What kind of fuel do you use? I had some problems when I tried to use a 10% ethanol fuel. To use leaner fuels like that you have to adjust the FQS (fuel quality switch) and richen the car up a little. As for the vibration, my guess is either motor mounts going out and just haven't alltogether went out or your balance shafts are not inalignment or the belt is broken on the ballance shafts. It's a really good idea to change the timing belt and balance shaft belts if you haven't already anyhow, so I'd check that for vibrations. Watch those dern vibrations, my motor mounts went out about two weeks ago and while I was waiting on my new mounts to come in my fuel rail got cracked due to the vibration...so that's always a pain. Let us know what you find out. I'm sure we can come up with some other sujestions if that doesn't solve any problems.

manu4jc · 08-24-2007, 09:13 PM

Thank you soo much. I will check the ground terminal on the battery tomorrow...is there another ground somewhere that I should check. I know my M3 has one on the engine too.

As far as the mounts I dont know if that is the case with me as the vibration is very light... slightly discomforting but not anything major. I was guessing Idle control valve...replaced it with a new one same thing. My next guess is the Mass Air Flow Meter.

bholmes · bholmes's Garage

Only reason I say mounts is because mine has a similar slight vibration that got catastrophically worse when the mounts went out all the way. They are a fluid filled rubber mount, so if only one goes bad, or just leaks some of the fluid out it's not as easily detectabled. But it could verywell be a sensor. As for the ground, I'd use a volt meter to check for continuity on both the frame and the engine, often times rubber mounted engines have a hard time with ground...especially aluminum block engines. There should be a ground strap from the block to the chassis I believe, most cars have them, I'm not sure where it is on the 944. I've never had to look. Well, good luck and keep us posted!

livewirevoodoo · livewirevoodoo's Garage

Search the forum, you should find a diagram of all the ground locations.

For the hesitation and idle.... Check vaccum lines, throttle position switch, idle control valve, afm (clarks-garage.com or frwilk.com has good write ups on how to refurbish afm wiper tracks). As mentioned, safe bet to change the fuel filter.

csmall · 08-25-2007, 02:59 PM

Heres something real simple you can try... unplug the oxygen sensor and see what happens. My 924 developed some serious hesitation problems and the idle was a bit rough. Unplugged the O2 sensor... just like that I have a nice smooth idle and no more hesitation. The O2 sensor was shot, had never been changed... I'm putting in a new one this week. I have to admit though, the car runs pretty good without it.

blk924s · 08-26-2007, 09:35 AM

There is an engine to chassis ground at the rear of the engine near the speed and reference sensors.
http://www.clarks-garage.com/graphics/speed-reference-sensor-2.jpg
This picture shows the sensors looking from the back of the engine forwards, so the ground is to the right just out of the frame.

Ernest Hill · Ernest Hill's Garage

check your fuel FLOW also (replace fuel filter regardles if you do not know how old, check fuel tank 'sock' filter while you are at it, see clarks-garage), but more likely check your ignition, sounds similar to a low spark situation, do you know when the cap and rotor were replaced?; & 2X on the O2 sensor advice