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81 930
 
Join Date: Sep 2007
Location: Slidell, LA
Posts: 12
Garage
Misfire

Okay guys, first real post but a member of the forum for quite some time. Just been reading and gaining knowledge from the posts. I have a 1981 930 which Iíve owned since 1996. Iíve performed all the required maintenance and a number of upgrades since 1996. Upgrades are;
1) SC Cams dialed in a 1.5 mm
2) K-27S Turbo
3) Tial 46 mm Wastegate with 1 Bar spring
4) B&B headers with muffler
5) TurboKraft Intercooler
6) Spec Stage 2+ Clutch with aluminum flywheel
7) Heads, injection blocks, and manifold ported to 36mm and polished.
Iím currently having a problem and need some advice. I have a new set of wires (Nology), new coil (Nology), and plugs (Denso Iridium IW24 w/ .28 gap). Timing is set at 4⁰ BTDC at idle with vacuum advance hose plugged.
Fuel pressure numbers are; 32 psi cold pressure/ 90 psi WUR blocked / 57 psi warm pressure (WUR connected). Front fuel pump is a new Bosch 044 with the original pump in the rear. Disconnected fuel injectors and installed individual bottles to check flow pattern and leakage. Injectors tested okay. Injectors have only ~ 1000 miles service from new.
Hereís the problem. The car idle is smooth but stumble/misfire somewhat when accelerating from a stop and then smooths out. Sometime itís smooth from a dead stop through the entire RPM range but not consistent. The idle is somewhat high after the car is warm and I could not adjust down lower than 1100 with the idle adjustment screw. I thought it could be and injector but donít think the injector would cause a high idle and didnít know if the idle and the stumble are related. Thoughts/recommendations appreciated. I will post pictures later. Thanks

Old 07-08-2012, 03:52 PM
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Join Date: Sep 2002
Location: Westchester County, NY
Posts: 1,918
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Nice combo and good troubleshooting to this point. The board is very helpful but it's nice to see some homework done once in a while, too.
Your high idle and stumble may be a vacuum leak or possibly a fault AAR valve. Search some more on how to troubleshoot it... it involves checking for 12V at the connector but then AAR removal, a freezer, and an oven.
ALso make sure every vacuum line is good, the woven-covered originals are 25+ years old now, and injector blocks get loose now and then too.
Lastly, the cap and rotor, and green wire on distributor (that might be brownish from heat now) can cause odd misfires too.
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Ken
1986 930, 1969 Mach1, 2003 540 Sport, 2016 R1200RS
Old 07-08-2012, 04:33 PM
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81 930
 
Join Date: Sep 2007
Location: Slidell, LA
Posts: 12
Garage
Thanks for the feedback. I'll continue to look for vacuum leaks. The wiring engine wiring harness with the green wire to the distributer is new but will double check the connection. I purchased new injection blocks and had the bored to 36mm. Torqued the injection manifold per specifications. I will double check manifold torque also. Attached are the pictures I promised. Thanks



Old 07-08-2012, 05:10 PM
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,289
I'd guess a vacuum leak that turns into a small boostleak when accelerating. They are hard to find. You can't see a dry rotted rubber crack in the origonal blue or red fabric covered vacuum lines until they pop under boost.
Boring out the oem bakelite or plastic injector block holes and gaskets from 32 to 36mm makes them alot more vulnerable to cracks, warping, and leaks. Much better to put aluminum ones in.

A wideband AFR gauge will help diagnose it.

oh, Nice car! Is it grey or dark blue grey?
Old 07-09-2012, 07:29 AM
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Join Date: Nov 2007
Location: Rockwall, Texas
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An unrelated observation - it looks to me like you could get an additional 2-3 inches of airflow to your intercooler by replacing the block-off plate (to the left of the a/c condenser) with an appropriately narrower one (I did this when I installed an aftermarket intercooler and built a custom shroud for it).
Old 07-09-2012, 07:37 AM
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81 930
 
Join Date: Sep 2007
Location: Slidell, LA
Posts: 12
Garage
Thanks guys. What AFR gauge do you recommend? I could really use one. I will look into installing a custom intercooler shroud once I get all the bugs worked out.

The car is a blue. Again, thanks for the input.
Old 07-10-2012, 04:08 PM
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89 930.
 
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Join Date: Jul 2008
Location: Canada Ontario
Posts: 783
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You could try the old school way. Chech all hoses with a unlit propane torch.When the propane enters if runs rough.

Old 07-10-2012, 04:43 PM
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