dizwiz24

I just did a compression check on my 93 LT1 engine. It has 48,000 miles. Many are hard miles. However the car has been serviced regularly (oil changes, etc.)
My mods include hotcam, ported heads, TPIS headers, Ed Wright chip.

Heres what the cylinders read... This is an average of at least four 7 second bursts of cranking on each cylinder.

Fuel pump fuse and spark coil disconnected. Engine was cold (I know some people insist you need to do this test hot, but I dont feel like getting burned)

Cylinder 1 (closest to driver on driver side)
185

Cylinder 2 (drivers side)
188

Cylinder 3 (Drivers side)
187

Cylinder 4 (drivers side closest to front of car)
187

Cylinder 5 (passenger side closest to front of car)
186

Cylinder 6
206 ?????? (keep in mind this is an average, all readings were right around 205)

Cylinder 7
191

Cylinder 8
181



Anyways, what is up with my cylinder 6? I should mention the spark plugs looked fine. Not excessively sooty, no oil, etc.

I am going to do a retest, but I anticipate the same results.

dizwiz24

I should add, what made me want to do this test was that I am burning some oil (about 1 quart every 1200 miles).

I get a puff of blue smoke sometimes at startup (not real bad though).

A small amount of oil residue can be seen under the open breathers of my aftermarket valve covers -especially after drag racing



I also need to describe the needle behavior. During cranking the guage would hit like 160 PSI after the first revolution. Then after the second revolution it'd hit either the final number, or 180. But always by the third revolution itd hit its peak number.

It would stay at its peak for at least 5 minutes...(I didnt check longer) and show no evidence of bleeding down.

tjwong

When I peform a compression test, I crank the engine over and let it hit the gauge 5x. Do that for all the cylinders. Number 6 may just have an excessive amount of carbon build up on top of the piston. That may explain the slightly higher reading.

8388

This is going to sound crazy, but I will sometimes spray a mist of water while reving the engine slightly. This will get rid of most of the carbon build up from the combustion chamber aswell as the top of the pistons.

I would also do this before disassembly of the engine. It leaves the top of the pistons nice and clean.

You might have excessive carbon buildup in that cylinder.

baumann95lt4

you should do a leakdown test on all of them. A compression check is should only be the beginning. Many problems can be found with a leakdown that will not be evident in a compression check.

tjwong

Not crazy at all I use GM TOP ENGINE CLEANER to disolve carbon build up. And in real bad cases, I spray water through it as well, the steam that occurs when the water spray enters the combustion chamber seems to blast the carbon away.

By the way GM TEC also makes a excellent firearms bore cleaner

dizwiz24

I find it kind of odd that it has carbon build up - but just in one cylinder.

A retest, confirmed the high number.

The retest for that cylinder gave - 220-220-205-205-209

an average of 212.


Doing a leakdown test is, of course, preferred. However, I dont have the gauges and air compressor to do it.

Just happens I had a compression gauge.

8388

Too much oil or fuel in one cylinder will do it.

dizwiz24

maybe there is a problem with that one injector then?

My gas mileage has been somewhat poor lately (15-16 mixed city.highway)...Only 20 MPG on the highway...

Keep in mind my mods also.

dizwiz24

Actually, is that one injector specific to that cylinder or is it part of a pipeline that feeds fuel to all cylinders?

MAybe the injector right over the intake port on the head for that cylinder leaks?

8388

Yes. Batch Port injection. 1st do a pressure test. If you lose pressure after priming the the system you have a leak. also inspect the spark plugs, see if its wet with fuel after priming the system.

93 ragtop

As far as the puff of smoke goes at start up, a lot of times that is bad valve seals. Now IMHO if one of the seals on the no. 6 cyl. is bad and letting a small amount of oil in, that would raise the compression on that one cyl. on a compression test.