airtraffic

Hi guys!

I have an '87 944 N/A. 77K miles. Runs great, but after an hour on the highway, if I pull up to a stop sign, the RPM's dive to about 200 and the car sputters and almost dies. If I give it a little gas the RPM's will come back up (after about 5 seconds) and stabilize at 800-1000 and I can go on my way. If I try to pull away from the stop sign without waiting for RPMs to stabilize, the car will buck like a bronco.

I suspected the DME could be a possibility and had the opportunity to pick one up fairly inexpensively, so I changed that out.. No change. I know I need to renew all my vacuum lines, but I'm wondering if the Oxygen sensor could be causing an overly lean condition after the highway cruising? Could this be causing a fuel starvation issue as I pull up to a stop sign? I have read that oxygen sensors are good for 60K miles, so I'm about 17K miles overdue. All tune up items, plugs, wires, dist cap & rotor, etc are good.

When the engine is not fully hot, I do not have this problem.. Only when it's heated up to full operating temp. If it's warm, like after startup and a few minutes of city driving the idle is pretty stable, and I don't have stoplight/stop sign issues. Perhaps the oxygen sensor is still signaling a richer mixture?

944kid

Number 1 diagnostic technique, always tackle one issue at a time. In other words, replace (don't patch) the vacuum lines first, and if the problem persists, then pursue other possible causes.

944kid

Also worth noting is, your car is an 87 with low miles. Drive it more! These cars suffer when they're garaged or driven spuratically, and consequently, the parts go bad. Vacuum lines are often assumed to be good for a lot longer than they are actually good. Primarily because the rubber gets dry rotted through the heat cycles and natural aging of the rubber. Which came to mind when you said that the car only does this when warmed up, which would make sense if there are tiny pin holes in the lines. Heat causes objects to expand, my money would be on the lines for sure, so definitely replace them and report back! We're here to help.

airtraffic

Thanks for the replies.. 944kid.. I'm not the one guilty of not driving it. That's courtesy of the previous owner.. But I thank him for leaving me a gorgeous, low-mileage 944S!! I put 10K miles on it in the year I have had it. I'll be working on the vacuum lines and increasing the odometer count.

BTW, if anyone knows of a good writeup for the 944S vacuum lines, I'd love one. I've seen the diagram that's posted on the boards, but that's just a copy of what's in the engine bay.

Jfrahm

What you have is more of a vacuum leak kinda problem, but IMO more of a problem than a small cracked hose could cause. A major leak from a cracked J pipe or ISV hose perhaps. The idle stabilizer might also need to be cleaned, they can be removed and de-carbonized with elbow grease and harsh chemicals. You might need an O2 sensor but they generally just cause you to run a bit rich and ruin your catalytic converter. The ISV can cause a bad idle but not the bucking, that is usually a leak of unmetered air. See also your TPS and your AFM, there have been threads recently on both including fixing up the AFM.

What is your location?

-Joel.

tod84944

Search on Clark's garage how to check the DME temp sensor. Also,make sure the TPS is also working properly. I have this issue come on on the 944's I service,and those are the first things I check. After that, check and clean the ICV. Once again, you can do a search on Clark's-garage.

airtraffic

Thanks! I'm working on getting the intake removed to get at the TPS and idle valve. I'll want to check the throttle body seals while I'm at it. I did have an intake manifold allen bolt head strip out on me, so I guess I"m drilling that one out. Ugh.

To make this a bit more interesting.. I drove home from work yesterday. 1 hour cruising on the highway just fine. Pull up to the 1st stop light, the car does it's usual rough idle/sputter thing. I give it gas and stabilize the motor. It stabilizes at 1000 RPM. I pull away from the light and proceed home through 2 more stop lights with no issues whatsoever. The motor does NOT sputter or cough at the remaining stoplights, but rather idles just fine at 1000 RPM.

tod84944

One more thing, on the 16V cars, I had to replace fuel pumps on 2 different 16V cars due to them loosing fuel pressure after running at highway speeds and then slowing down.

airtraffic

"on the 16V cars, I had to replace fuel pumps on 2 different 16V cars due to them loosing fuel pressure after running at highway speeds and then slowing down"


Hmmm.. Very interesting!

airtraffic

Joel,

Thanks, I have looked at the AFM and and going to look at the TPS as well. I've read the threads on both of those. I'm in Eastern PA.

airtraffic

Tod84944,

How would you be able to tell if the fuel pump had slowed down after highway driving? You would not be able to measure pressure or flow rate at that point would you?

mytrplseven

Are you an ATC specialist? Just curious from your name tag. Retired AA pilot myself. Anyhow back to your problem. I have this problem on my Dodge truck and after extensive research found that the ISV (idle stabilization valve) is the culprit. Dodge knows about it and hasn't done anything about it. I suggest you check the vacuum lines to and from the valve and if necessary, pull it, clean or replace it and I think you'll be ok. Good luck in the New Year.

tod84944

I put a fuel gauge on the rail. I would open the hood and check pressure before interstate driving and after.

gtroth

Idle flop on the S sounds like ISV sticking to me (they can get kind of gummed up). 16V engines have dynamic idle stabilization and if the valve can't respond in time rpm dips below its target 850rpm, and DME does what it can to prevent stalling.
You can clean it, but you really need to remove it to do that (remove intake manifold...).
I replaced mine, and that cured my idle flops.
I cleaned the ISV on the Carrera since its right on top and that worked too for that car, fwiw.

airtraffic

Thanks guys!

Yes, I have been meaning to get at the idle valve beneath the intake manifold.. In fact, I was in the process of removing the allen bolts for the IM when one of the heads stripped out.. Sooo.. Now when I have time to get that stupid bolt out, work will continue. ICV and vacuum lines. I'm also considering replacing the fuel pump and filter, as I think I have a line on an inexpensive Bosch pump.


Trplseven, Yes, I'm a controller at Newark Airport.