Too Much Compression ?

[vol96]

I just read on the ngk site that too much compression combined with incorrect plug gap can cause missfires. Since my motor was rebuilt its compression is 190 in all cylinders. I'm running ngk bpr7es gapped 030.

Car seems to run good at WOT, but misses when cruising and has low vacuum at idle (about 11inHG).

I have been looking for vac leaks and have fixed the easy ones (tb gaskets, bisss oring, vac lines). Before I get into the harder ones (Iman gasket, valves) I'm just thinking maybe my high compression is causing misfires?

BTW, compression is hi cause I had the head milled to get it flat when I rebuilt the motor. Car is 95 TSI bone stock (except for milled head).

 

[Demon33]

Your plugs might be too cold. Try the BPR6ES plugs gapped to 0.028". They are one range hotter and will help ignite the air/fuel mixture better on an otherwise stock engine like yours. An added benefit of the hotter plug is that they will reach self-cleaning temperature faster, reducing the chances of fouling. Generally, the BPR7ES plugs are used in higher boost (>15psi) applications.

 

[vol96]

So you dont think my high compression is a problem for the ECU?

 

[antilag]

compression is not your problem. My compression is 210 across the board and no problems with ngk's. It sounds more like a leak to me. I would also go back to the 6es as stated above.

 

[vol96]

antilag - thanks for the input. I will continue to look for leaks.

 

[vol96]

Went back to 6es. Still missed under wot. Checked all sensors. Found CTS was about 500ohms too low. Put in new cts, now no miss under wot. But still have low vacuum.

At idle I can hear faint and steady hissing from the big part of the intake manifold (plenum) between mpd sensor and fpr port. But cannot pinpoint the source. Not even sure this indicates a problem.

 

[Rob10_99]

Boost leak test and find the source.

 

[vol96]

I have done a boost leak test, several times actually. Mostly from the elbow. Got all my vac lines plugged except fpr, bov and pcv. Even plugged them during boost test and still cant find the problem. Its probably staring me in the face but I cant see it. Replaced biss oring, tb shaft seals, tb gaskets, FI insulators.

 

[Rob10_99]

Do it from the turbo inlet, pump it up to 10-30psi and wait for it.

 

[vol96]

I have done it from turbo inlet after I fixed leaks at TB. Found no leaks in turbo plumbing. But there was gurgling from the block and engine smoked like a b on next startup.

Dont know why it smoked.