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Unhappy Stubborn miss at low RPM

My 86 turbo has a miss at low rpms. I replaced plugs, injectors, filters, fuel pumps, wur. Fuel pressure is right, timing and valve adjustment done. Iím sure itís a vacuum leak. Any safe suggestion to figure out exactly were it is ?

Last edited by spool930; 12-04-2017 at 06:07 PM..
Old 12-04-2017, 06:03 PM
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If you enrich the mixture does it go away?
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 12-04-2017, 06:14 PM
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If you pull the plugs after it runs for a short while and its still missing, can you tell which cylinder is missing? (darker color) This will help to focus your investigation.
Old injector blocks are notorious for cracking and leaking. Plus be sure that all the nuts holding the intakes and injector blocks tight to the heads are properly torqued. They are a common issue.
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Old 12-04-2017, 06:24 PM
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I agree with jeff just dont over tighten the nuts you don't want to strip em..

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Old 12-04-2017, 06:37 PM
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and if that isn't it.. the fuel head might have an ethanol hangover and now one cylinder is getting too much gas at low rpms.
The original rubber diaphragm and rubber o-rings in your 1986 aluminum fuel head are not ethanol tolerant.
Old 12-04-2017, 10:29 PM
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Another idea:
A leak of air of the servo brake can also be the cause of a bad slow motion, in this case it will be necessary to replace him(it).
Old 12-05-2017, 04:36 AM
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is the O2 still connected?
ignition advance working properly.
mixture set correctly
how is the idle

18ft lbs torque. I had issues with mine and with them coming lose. also had issues with the intake sealing to the blocks. basically the in take surface was concave causing the injector blocks to do the same. also if the intake gaskets are not put on the in the correct orientation they can leak.

check plugs as suggested. I would check at idle if it idles bad too.

once this is done you might consider comparing fuel flow from the injectors. this is a real pain.
might be easier to check out of the fuel line to the injectors first. try not to bend the lines.
just hold the sensor plate down about 1/2 inch to simulate your RPM issue.

other option is to just rebuild it.
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ [__] RUNNING: [__] NOT RUNNING:
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Old 12-05-2017, 06:28 AM
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Thanks for all the help.

RarlyL8, yes it does improve whe I fatten it up a little, I also checked intake bolts when doing the injectors.

The bad plug was #3.

Idle is when it misses, runs great the rest of the time.
Old 12-05-2017, 07:42 AM
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No body has mentioned the green wire yet. My 86 had a badly cracked green wire that caused some occasional missing at low rpm.
Old 12-05-2017, 07:45 AM
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Quote:
Originally Posted by spool930 View Post
Thanks for all the help.

RarlyL8, yes it does improve whe I fatten it up a little, I also checked intake bolts when doing the injectors.

The bad plug was #3.

Idle is when it misses, runs great the rest of the time.
Are the injector blocks original? If so, good chance they are cracked.
When I replaced mine with TK's injector blocks, I somehow didn't screw in one off the injectors all the way. It was most of the way in, but in an effort to not overtighten, I didn't realize that it was not seated tightly onto the block. That's all it took to allow a bit of air to get by and cause a miss at idle.

I believe Fairman did a flow test of the injectors and one of his leaked when they all should have been closed. He has a great thread on here where he documents how he discovered it and how he rebuilt it the fuel head.
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Old 12-05-2017, 08:20 AM
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Originally Posted by flightlead404 View Post
No body has mentioned the green wire yet. My 86 had a badly cracked green wire that caused some occasional missing at low rpm.
Would that cause misses on random cylinders or cause it to be primarily on one cylinder? Maybe both are possible?
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Old 12-05-2017, 08:22 AM
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If the mis is only at idle and it goes away when you enrich the mixture the problem is not ignition related.
Did this problem develop on its own or after an action was taken?
Do you have an AFR meter?
If you do have an AFR meter installed was it calibrated recently?
Is the Lambda system intact and functioning?
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Last edited by RarlyL8; 12-05-2017 at 08:35 AM..
Old 12-05-2017, 08:33 AM
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Green wire is good, injector blocks are original. Afr is 13.9 @ idle, gauge not recently calibrated, lambda is disconnected.

I had a couple of leaky injectors, thatís why I replaced them all. I did not check the new ones.

I had a fuel pressure problem, the car was running very fat, it was resolved by new filter and fuel pumps. Thatís when I added AFR. This resolved the rich condition, but now I have the idle problem.

Iím pretty sure itís a cracked injector block or the fuel distributor.
Old 12-05-2017, 09:42 AM
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The leaking injectors could have been caused by bad injectors, or by the fuel head being shot as described by Fairman above. See if they leak now, and if they do, swap them to a cylinder that isn't leaking and see if the leak follows the injector, or if is stays with the cylinder. If it stays, fuel head is a good bet.

Pull #3 injector block and carefully inspect it. Knowing they were a common failure point and that mine were original, I replaced mine with TK's when I pulled the motor a couple years ago.
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Old 12-05-2017, 09:51 AM
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Yes, strong luck(chance) whether it is blocks injectors, especially if they are of origin.
Old 12-05-2017, 10:15 AM
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water will find and air leak.
use a spray bottler to spray around the blocks.

smoke test

was #3 black?
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86 930 42kmiles [__] RUNNING:[__] NOT RUNNING: ____77 911S widebody: SOLD
88 BMW 325is 200K+ [__] RUNNING: [__] NOT RUNNING:
05 BMW 330CI 130K:: [__] RUNNING: [__] NOT RUNNING:
08 VOLVO V70 190K:: [__] RUNNING: [__] NOT RUNNING:
90 B2200[] RUNNING:[_] NOT RUNNING:__2000 MER E320 WAGON [_] WRECKED:[]RUNNING:
Old 12-05-2017, 10:20 AM
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I would first calibrate the AFR meter and verify idle mixture.
If idle mixture is 13.9 that is too high/lean, bring it down to 13.0-13.5 and see if the lean pop goes away (cruise should be 14.5-14.7). If not then time to look at injector blocks.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 12-05-2017, 10:32 AM
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Thank you all for the help. I will calibrate the afr gauge, set the idle at 13.5. And use a spray bottle to see if I can find a air leak. If that falls Iíll pull the injectors and see if the fuel supply equal.

May be time to just pull the intake replace the injector blocks and have the fuel head rebuilt. The motor was rebuilt a few years ago, but, the injector blocks fuel distributor are all original.

I can get injector blocks from TK, who would I go to for the fuel distributor rebuild?

Last edited by spool930; 12-05-2017 at 05:38 PM..
Old 12-05-2017, 12:20 PM
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Quote:
Originally Posted by spool930 View Post
Thank you all for the help. I will calibrate the afr gauge, set the idle at 3.5. And use a spray bottle to see if I can find a air leak. If that falls Iíll pull the injectors and see if the fuel supply equal.

May be time to just pull the intake replace the injector blocks and have the fuel head rebuilt. The motor was rebuilt a few years ago, but, the injector blocks fuel distributor are all original.

I can get injector blocks from TK, who would I go to for the fuel distributor rebuild?
Don't rebuild something that may not need it. While some people have pulled theirs open to find that it is shot, mine has never been touched and the fuel metering out of it through my 30yr old injectors is perfectly even and in spec. Test it before spending money on it and then spend the money on things you know you need.
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Old 12-05-2017, 12:50 PM
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Thanks
Old 12-05-2017, 01:22 PM
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