Hi All,

I need some insight here. I am running a completely bone stock '86 951. I was driving on the freeway today and noticed a problem. The engine will jerk whenever I tried to accelerate. I did this a few times, and noticed the following:

* Boost gauge still shows 2 bar when jerking starts
* Jerking typically starts at around 3,200 to 3,500 rpm or 80 to 85 mph

The jerking feels like a starvation problem in the engine. The jerking will go away when I let off the gas. The engine will run smoothly again below the 3,200 to 3,500 rpm.

I do not think it is turbocharger problem because the boost gauge was still registering a full 2 bar when the jerking started. I am wondering if it could be related to the rest of the turbocharging system (hoses, valves, etc) or the fuel system.

Anyone have ideas for a trial and deduction method for diagnosing the problem?

 

Vacuum lines maybe?

 

If the car is bone stock, your gauge should not show 2 bar. (Which would really be 1 bar, = 14.5 psi or so.)

 

The jerking could be the result of an over boost condition. The KLR/DME will limit ignition if excessive boost exists as you explain this happening only after gauge hits 2 bar.

 

I had suspected an overboost situation, but can it occur at 3,200 to 3,500 rpm? If this is the case, any ideas on what could cuase this?

 

Hmm. Strange, on Sunday my buddy and I were driving my 86 951 down to the border to look at my 968. the 951 started bucking /cutting out at normal speeds 70mph on the highway, so we pulled over to find out why. It stalled on the road and I had to have my car towed home because I could NOT re-start it!!! It looks like one of my injectors was leaking past the fuel rail, and it looks like there is a lot of oil leaking below that injector on the exhaust side around the headgasket.

Maybe we have similiar problems?

 

"* Boost gauge still shows 2 bar when jerking starts
* Jerking typically starts at around 3,200 to 3,500 rpm or 80 to 85 mph"

Uh... your boost is way too high for a stock car and you're hitting the overboost protection cut-off. Stock boost should be 1.6-1.75 bar or so. Check the banjo-bolt to make sure it doesn't have a restrictor-jet screwed into it. Check all the hoses from the banjo-bolt to the wastegate. If there's a leak in any one of them, you'll be venting pressure that should be going to the wastegate to open it to reduce boost. Could also be the plastic ports on your CV-cycling valve, these like to break A LOT and even just replacing a hose can break the damn thing!!!

 

Thanks for the tips guys. I recollect taking the car to the shop a year ago. The shop fixed a leak in the oil pump assembly area. I am wondering if they may inadvertently knocked off a hose or something. I did not get a chance to drive her much after the job was done. She went into storage as I had to leave for Singapore for my assignment. I have a couple of days before I head back to Singapore. I'll take a look under the hood again with your tips in mind.

 

Sorry to hear about your problem Rich. It's great that you have a "spare" 968 to drive while you fix the problem on your 951 huh?

 

Ever replaced the Throttle Position Sensor?

Had the same symptoms in my S2 and a new TPS fixed the problem for good.

 

I ran into this post while searching for answers to a similar question.
Last weekend I ran a SCCA drivers school at Lime Rock and my 86 951 developed similar symptoms as Andrew's. In addition I noticed a couple of other things happening. While bucking and cutting the call would backfire. This leads me to believe that there was unburned fuel in the exhaust system. Also the cutting off will start at about 1 bar (3-3.5k RPM) and the boost will go (too) quickly close to 2 bars if I stayed on the throttle. Other than that, the car ran OK and I was able to nurse it through the school and get signed off to race.

One additional vital piece of information. I had the rod bearings replaced and the turbocharger rebuilt prior to running the school and no time to go over the car carefully.
Thanks in advance of your suggestions.
Anthony

 

Okay, ACG start simple and work up to overboost and vaccum problems. Start first with your baterry cables. Chase them all the way, pos and neg. IF that doesn't turnout to be the problem, check you4r fuel filter. If original, change it out anyway. It's got crap in it.
Now how sure are you that the gauge ACTUALLY said 2 bar. Betcha it was about a line and a half or two below.
If, as happens all the time with these cars, your pos or neg cable have had the insulation burned through, they will move under acceleration, and arc on any number of possible areas between the engine and firewall, but only until the car settles back into a "normal' level of rear squat. Don't chase the big stuff first. You will waste your time more often than not. If you can't get under and back there, then take it in and have it checked. It won't cost more than 80 bucks, and that's if the joint is run by a jerk.
If not there, than when was the dist. cap last checked? etc etc. Start on one side of a system and work systematically, don't just jump around. Ask me how I know