My friends '87 924S has been bucking randomly. It happens out of no where while he is in gear cruising. To get it back to normal he lets off the gas, and it drives ok. If he gives it more gas when it happens, it will not rev, but buck harder. Its almost like it is just cutting out for a second, then bucking. Anyone have any idea of whta it could be?

Thanks
Mike

 

That's weird. My 914 did the same thing, but I sincerely doubt for the same reason. My reason had to do with either the ignition or the amount of fuel at any given time (manifold pressure sensor, which you guys have MAFs I think?). I really can't pinpoint it though because I rebuilt the engine and changed the distributor. In my case it looked like it was skipping and running very erdadically.

 

Mine does that. DME relay replaced lately? could be a ground, or something.

now I'm not too sure YET but I'm thinkin the Oxy sensor may have fixed it for me. I'd say get a dME relay, if that doesnt' work, an O2 sensor, and maybe add a supplemental ground.

 

OK, thanks, I think he has a new DME relay, but hasnt installed it yet, so I'll tell him to give that a try.

 

AFS. See Wilk's site about how to clean it.

www.frwilk.com

 

U sure SoCal?

 

Clutch rubber damper center going bad, combined with certain technique with the gas, can cause bucking as well.

What gear are you in when this happened?

 

Pretty bold.. I dont question SoCal- the answer is usually correct


But I gotta throw my .02 in.. Maybe bad gas?

 

Check your vaccum lines - I had the same problem recently on my 944 and it was a fractured vaccum tee (the plastic got brittle and cracked). $1.49 worth of parts and runs like a top.

 

Yep.

Is your friend a member of Rennlist yet??

Sorry- damn evangelists. It's my lot in life. I have tagged 9 cars since being in Ohio, some of those people are lurking now at Rennlist. So far no members.

<on topic>

Vacuum lines are cheap and relatively easy to replace all at once. I recomment it highly.

Is it an automatic? more info please Mike.

 

It happens in any gear, uaually 1st and 2nd, and its not automatic.

 

Even with a new clutch disc (which would have a new damper), I can successfully "buck" the car.

Maybe letting off the gas in 1st gear coming to a light, stepping on the brake (pretty hard?), don't come to a full stop, then back on the gas 1/2 way, then let back up. The car will keep bucking until you let off the gas fully or push the clutch pedal in.

I think it's a question of driving technique. I never do it now, but I used to when I got the car, and other people who drive the car have this happen to them.

It never happens to me when I am driving at a steady speed in 1st or 2nd gear, with the gas pedal held steady.

 

What are the engine systems that can cause this symptom of "bucking in any gear"?

If the ignition is cutting out it would do this all the time not when the gas pedal is pushed down (acceleration).

If the fuel delivery is off (fuel pressure) then this could cause the lack of proper fuel delivery when the gas pedal is pushed down. Usually a fuel pressure problem will continue to happen even at an idle.

So what is the only other active sensor that is involved with the demand for acceleration (pushing the gas pedal down)? The air flow sensor.

If the AFS is dirty or has a worn resistor surface, the info it sends to the DME is corrupted. GIGO. Cleaning it and relocating the wipers is a fairly simple job. Twenty maybe thirty minutes at the most. This will eliminate the AFS as the source of the problem. If it was the AFS then it's like having a new car.

Yes this could be a vacuum leak. The large rubber duct between the AFS and the throttle body could be cracked of have come off of one end a hair. Again this impacts the function of the AFS.

Could be the DME relay not keeping the fuel pump running. Put a test light off of Aux fuse #2 to ground. If the DME relay is acting up you will see the light go out before the bucking.

Could be a dirty fuel filter. Or a bad fuel pressure dampener.

Have to start somewhere.

 

Well, here's the thing. for me, I DID replace the DME relay. It does it only when it's a bit chilly out. It's not running lean, so It's not a vacuum leak, and it happens in any gear, but I've noticed it most in 5th gear when I'm cruising at highway speeds. no accel or decel. holding a constant speed, the engine will cut out for a moment, I let off the gas and hit it again, and it picks up again. it's usually accompanied with a bit of a clicking sound from somewhere near the gauges, and it happens when the headlights are on. This is what leads me to beleive it's a bad ground somewhere and running a supplemental ground from the negative post on the battery to the hoist point on back of the cam tower would help it.

SoCal, does this sound kinda like the AFM bein dirty?

 

one of the first things I did was to completely resolder thr DME. I havent had a DME problem since. and it's been 6 years.

I strongly recommend this. It took me about 3 hours. I can turn one around in a day if you want me to do it.

If it were FPR it would be difficult to start hot as well. these conditions dont exist.