924Board.org

GeneT95 · Posted: Wed May 05, 2021 11:00 am Post subject: CIS pressure tester

81 931. Runs well but occasionally with boost, when i really hit it to get it at 'full throttle' in first and second - daily, it bucks and misses. Not all the time. Seems related to fuel delivery as it did this a while back but I thought the fuel pump was bad - got noisy -and replaced with resolution. This is an intermittent problem. Usually when I jump on it, it spools up with no problem straight to 5k. It also runs fine at moderate spry driving just not 'full throttle' during those smile inducing occasions. It does have a subtle cold start issue - initial stumble, slight tap on gas, settles to normal idle. This is worse in cold weather and only on first morning start or after sitting.

Need to check a lot of things for general maintenance and looking for 'correct' CIS pressure tester as listed in "Systematic Approach to 931 diagnostics" but obviously finding 'that' one is difficult.

Which CIS pressure tester should I get and from where? the 33800 or 33865? I just don't want to get the wrong one and fumble around with it as the 'Diagnostic' posts seems to indicated I need the 'correct' one for a 931. I don't mind spending the money, just not on the wrong tool.
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61 Biscayne : 62 Bug : 81 931 : 88 944 NA

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

Could it be overboosting and tripping the fuel cutoff switch?
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

Fasteddie313 · Joined: 29 Sep 2013 Posts: 2595 Location: MI

Either should work fine..
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80 Turbo - Slightly Modified

GeneT95 · Posted: Wed May 05, 2021 10:52 pm Post subject:

Possible. I thought of that the first time and recheck. But I believe I put a new one in and it didn't make a difference.

As it is intermittent and only with a heavy pedal, I thought the switch could be it. Maybe I'll have to replaced the wire or at least track it all down.
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61 Biscayne : 62 Bug : 81 931 : 88 944 NA

GeneT95 · Posted: Thu May 06, 2021 2:02 am Post subject:

Hmmm. Guess I will have to track the whole Overboost switch.

I drove it today. Was perfect. Started right up. Idle straight to 950 or so. Warm up. Moderate driving with boost to 7.5 on gauge. Heavy peddle with boost to 7.5 during two spirited take offs to 5.5k with no trouble in first or second. I generally have to stop after second least I get in too much trouble.

I can't imagine a vacuum leak or fuel delivery issue to be soo sporadic?? Can it overboost yet show only 7-7.5 on the gauge? It's stock with rebuilt Engine/Turbo etc with less that 6k or so.

When it does it, it's like a jolting kick in the pants, once or twice and I immediately lay off the gas and it stops and runs fine.

@FastEddie - Thanks.
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61 Biscayne : 62 Bug : 81 931 : 88 944 NA

Fasteddie313 · Joined: 29 Sep 2013 Posts: 2595 Location: MI

Does your upper charge tube have that ground strap to the coil mount on it?
Upper charge tube needs to be grounded in order for the overboost switch to ground through it..

You could just ground the wire that goes in the overboost switch to rule it out..
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80 Turbo - Slightly Modified

Rasta Monsta · Joined: 12 Jul 2006 Posts: 11733 Location: PacNW

I drove with mine grounded for years. Probably not a genius move, but if you have a boost gauge...
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Toofah King Bad

WeiBe (1987 924S 2.5t) - 931 S3

GeneT95 · Posted: Thu May 06, 2021 8:47 am Post subject:

My upper charge tube does not have a ground strap that I can tell. I'll ground the wire to test and ground the charge tube.

I assume I need to ground from the green circle as mentioned in the post below to the negative terminal on the battery yes?

http://www.924board.org/viewtopic.php?t=44710&view=previous&sid=235f1a7dbbc75393f1e55cca8b24ca10

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61 Biscayne : 62 Bug : 81 931 : 88 944 NA