mikelambert1987

Hey guys purchased myself a 1988 911 turbo, it has a redone protomotive motor with 53k total on the odometer. Car is running great except for two issues. One the motor only has around 2-4k mileage wise since rebuild. In the last 3k I have run the car it burnt through a little less then 1quart of 20-40 motor oil should I be concerned? Second and this is the one that is confusing the hell out of me and my mechanic. from 4xxx and up rpm wise the car will stutter. Not stall or die, but stutter as in still rev and pull strong but it almost sounds like the vehicle is backfiring really quickly. It only does it every couple runs and only when going from low rpm WOT all the way up until you let off the gas. Sometimes it will pull smooth and without issue. Where could I start diagnosing the issue. Car has CIS fuel system.

Mike

mikelambert1987

Also I checked out this list
http://www.pelicanparts.com/911/tech...oubleshoot.htm
Under the list of backfires under load would any of those apply to my car.
Thanks again,
Mike

Mark Houghton

I don't know the details of a Protomotive motor, so will just generlize:

The oil use is in the normal range. I wouldn't be concerned with a quart in 3000 miles.

As to the stutter/miss, I'm sure you know there could be many causes. Fuel or ignition, of course. Have you looked at the plugs to see how they're burning? Plug wires and boots secure and in good condition? Distributor rotor and cap in good condition? All vacuum hoses connected and not leaking? Are you running with the stock CDI ignition or aftermarket? And make absolutely sure your ignition timing is correct at 4000 rpms, or you risk too much advance at full boost = detonation = pile of molten metal.

Fueling: I would strongly suggest that you do (or have done) a control pressure check on your WUR (cold, warm, and at boost pressure). Could be your AFR's are out of whack and (God forbid) leaning out at full throttle. I'm hoping the noise you're hearing isn't detonation but just a simple miss.

mikelambert1987

Mark,
Thanks for the tips, I am definitely fearful of detonation. The biggest problem I have is that its an intermittent problem and more difficult for me to track down. It never does it when the car is sitting and you just rev the motor. I will start by checking the plugs and vacuum lines. The only reason I am leaning towards it not being detonation is that and I am embarrassed to say the problem has been on and off since i bought the vehicle 3 months ago. As I have been sorting the car out the detonation would have already done I would think serious damage and it hasn't seemed to be any problem the car runs great and pulls incredibly strong except when its pulling this backfire stutter noise.

Mike

Mark Houghton

does it smoke like a pig when it does that? You know, black rich smoke indicative of over rich? If so, could be the WUR under boost isn't functioning properly....dropping control pressure too much and screwing with your air/fuel ratios to the excessively rich side. One step at a time, see what the plugs tell you, and go from there.

JFairman

Try retarding the ignition timing a hair.
Loosen the nut holding it and turn it counterclockwise a hair and retighten the nut.

See if the occasional acceleration backfire goes away after doing that.

If it's misfiring and bucking as turbo boost comes on with dark smoky exhaust than it's running too rich as boost comes on.
That could mean a warm up regulator gone bad or maybe your engine has a modified fuel head on it and it's giving too much fuel under boost.
If thats the problem an adjustable warm up regulator can usually correct that by raising control pressuer under boost.

mikelambert1987

Pulled the plugs last night after work. They looked relatively clean. Also after really driving the car and narrowing down when this stutter backfire thing happens it is only at wide open throttle or very close to wot any other setting or just brisk acceleration to higher rpms is fine. Also it doesn't let out any smokey exhaust when it does this. I mean it could be smoking a little but since it only happens when under load at wot by looking out the back and having my friend behind me there is no undue amounts of smoke coming out. So retarding ignition timing this is where I get a bit out of my league. I am willing to give it a shot but if anyone could maybe point me in the right direction as to how to do this. Loosen it and turn it down a small amount where is it that I am loosening and moving?

Thanks again guys for the patience,
Mike

911rudy

It is a little thing but if you have one plug wire that breaks down or grounds out, it will usually happen under load and at the higher rpm. Since its only one plug and even then it fires 99% of the time it will feel like the engine is out of balance or vibrating just a little.

mikelambert1987

Best way to test for that other then visual inspection is the water spray at night right? where you spray round the plugs and look for arcing or ?

A930Rocket

I don't think I would spray water on the plugs/plug wires...

In the dark with lights out, just look at the engine and see if there is any arcing going on. If you don't know the last time the plugs, wires, cap, rotor, gas filter, etc. were changed, then I would change them all.

Set your timing with a timing light. Loosen the dizzy nut enough to be able to rotate it, then run the engine up to 4k RPM. Check your timing. Should be 28* IIRC with the dizzy lines plugged. Tighten the dizzy and check again.

mikelambert1987

Sounds good I am going to see if I can borrow a timing light from my friend or from an auto store. I don't think it would be plugs/wires cap etc since the engine was just rebuilt 2k miles ago but their is always the possibility.

87 930

Mike; Your oil consumption is quite normal. I like everyone's ideas. Check the condition of the green dizzy wire and your overboost switch/wiring/connection on the intercooler also.
I've sprayed water on COLD engines before and found many electrical faults; I believe that it's a worthy test.

Scott

mikelambert1987

Yeah I'm just working up the nerve to do the timing change, should be interesting. Green dizzy wire I will check that out, where is the switch wiring connection just sitting on the intercooler downpipe?

mikelambert1987

Also don't know if this means anything or not but my 1980 sc doesn't do this. When I shut the car off the motor runs a bit by itself. Not long but definitely enough to notice that it is going after the key is off and out. maybe 1- 1.5 seconds

Mike

JFairman

There is an ignition shutoff delay relay on the rear electrical console. it's the smaller of the two aluminum colored ones. The bigger one next to it is for the rear window defroster.
It leaves the ignition powered on for a couple few seconds after you turn the key and fuel pumps off so it burns off any fuel that is still in the intake ports or dribbleing out of the CIS injectors before accumulator pressure drops enough for them to fully close.

At least you know it's working.