RobbieB

Hi guys, I'm having an interesting problem with my '87 N/A 944. After driving it for 15 to 20 minutes, it starts to run rough. It doesn't run rough in the traditional sense, because it doesn't misfire and seems to accelerate just fine. However, there's a noticeable increase in vibrations that shake the shift ****, dash vents, instrument cluster, etc. as you go through the gears. On the highway and when it's idling, the vibrations pulse for around one second at one second intervals.

Initially I assumed this was a temperature related issue. However, if the car is shut off and restarted, the vibrations disappear and it runs great for 5-10 minutes, until the issue comes back again.

Currently I'm thinking it might be a problem with the DME, but that's only because I'm an idiot and have no idea what else it could be. If I can't figure it out I can always take it to Schneider's Autohaus here in Santa Barbara, but diagnostic time can rack up pretty fast. Fresh parts on the car include the intake boot, small vacuum lines, and DME coolant temperature sensor.

Anyways, thanks in advance for any input guys. Hopefully y'all can help me get this sorted out.

-Robbie B


Edit: Added video with description:


Video Description:

Video of my 944 idling and being restarted. It runs rough causing the audible vibrations in the dash, but runs smooth after a restart.

Details:

-It takes about 10 minutes after a cold start for it to run rough, and about 5 to 10 minutes after a warm start.
-When rough, it idles at around 1100 RPM, and when smooth, it idles at a little under 1000 RPM.
-When it's running rough, it's not just at idle; it's noticeable during street and highway driving.
-It runs smooth at higher RPMs.
-The problem occurs whether the O2 sensor is connected or disconnected.
-Checked for vacuum leaks by pressurizing intake and spraying soapy water on all vacuum hoses and connections

Recently Replaced Parts:

-Small vacuum lines
-Intake boot
-DME Temperature Sensor

mytrplseven

You may want to consider the engine mounts.

PaulD_944S2

Here's a thought from my distant past:

My wife had a Datsun B210 that would shake (badly) just slightly above idle when accelerating, almost like it was going to die. I probably wasted a couple of Saturdays looking for the problem without results. One day I happened to press on the air filter while it was shaking and the shaking stopped. Turned out, there was a vacuum line that had a crack in it and when the engine resonated at a certain speed above idle it caused the crack to open and close and make the engine shake.

So the take-away is to check all of the vacuum lines. Not saying that will find the problem, but when you are diagnosing weird problems, you have to consider everything. Or the motor mounts. Good luck!

951Dreams

While exploring out in left field ideas, maybe something with the balance shafts? Maybe something (belt, shaft itself, something else) gets out of whack after a bit (maybe when hot?), and corrects itself when shut down. Then works itself back of whack after a bit?

I noticed when I removed my belts that the balance shafts on mine were a bit out of alignment. Maybe a tooth or two? I'm going to guess, since someone took the time to mark them all up, that when they had installed the belt, it was aligned, and had moved out of alignment over time. Belt seemed really loose to me as well. Maybe when your belt gets hot, it's moving enough out of alignment that it shakes??? Just some thoughts....

odonnell

If it revs out ok when the problem isn't happening, don't think it could be balance shafts...thermal expansion isn't that drastic in this case. Robbie, does the engine idle at a higher speed when cold? A slower idle speed (when warm) even by 200 rpm affects vibrations significantly, at least on my balance shaft delete engine. Cross your fingers that it isn't something in the clutch assembly that heats up with use.

joes

Just for fun look and see if you balance shaft belt is still on.

951Dreams

Do I smell experience, and a good story here?

RobbieB

Hey guys. First off, sorry for not responding earlier, but I really do appreciate all of the quick and thoughtful feedback you guys gave a couple weeks ago.

Due to the nature of the problem, I don't believe it's related to the balance shafts, the motor mounts, or the clutch, but I could be wrong.

Anyways, to give you guys a clearer picture of the problem, I took a quick video (40 seconds) of the 944 showing it idling rough and then running smooth after a restart.


Video Description:

Video of my 944 idling and being restarted. It runs rough causing the audible vibrations in the dash, but runs smooth after a restart.

Details:

-It takes about 10 minutes after a cold start for it to run rough, and about 5 to 10 minutes after a warm start.
-When rough, it idles at around 1100 RPM, and when smooth, it idles at a little under 1000 RPM.
-When it's running rough, it's not just at idle; it's noticeable during street and highway driving.
-It runs smooth at higher RPMs.
-The problem occurs whether the O2 sensor is connected or disconnected.
-Checked for vacuum leaks by pressurizing intake and spraying soapy water on all vacuum hoses and connections

Recently Replaced Parts:

-Small vacuum lines
-Intake boot
-DME Temperature Sensor

Thanks again for any additional ideas!