So I just did a compression check because I was getting a misfire on cylinder 7 with br7ef's installed. The compression check came out at 130psi accross the board and 80psi in #7. The engine is a 98 stock long block and just has your basic bolt ons with only 80k on the clock. What could have caused this? It was tuned after I installed the bolt ons. Could it be something besides the rings or ringlands?
The car still runs great with the factory heat range plugs. It doesnt smoke and runs awesome at the track. I just dont get it. Any insight would be great.
Thanks.

 

I would double check the readings with another gauge, 130 is too low let alone the 80.

Why are you using br7ef's on a stock engine anyways?

You should have a much higher compression reading, more in the 180-200 range if everything is healthy.

 

Stock engine should be in the 150-160 range. Something very wrong or you are using the gauge wrong. You wll need to use a leak down tester to see where the compresion is leaking.

 

I thought that was way low myself. We switched to the colder plug cause I was getting to spray a 100 wet kit through it and if fuel was good work up to a 150. My problem is something is going on in cylinder 7 cause it was misfiring with the colder plugs at an idle but was fine under load. We changed plugs wires and even coil packs and the mis stayed there. Put back in the stock plug and it goes away. Ive called around and asked some sponsors about this and nobody really has a clue.

But before I go building another motor I would like to have some more info on what else I can trouble shoot to find out whats wrong.

 

Hate to question your compression checking ability, but did you forget to hold the throttle wide open when checking compression?

 

Do a leak down test....honestly i'll bet number 7 is toast if you were spraying. Number 7 runs lean to start with, usually tune a little extra fuel to be sure 7 and 8 live.

 

LOL I havent even sprayed it yet... we were gonna take it to the dyno to tune it for the spray but it was mising at an idle and we havent gotten past that yet.

As far as how the test was done I am not sure. I am deployed and my car is being worked on while i am here. The guy has quite a bit of experience with fbodys and ls based motors as well as any regular joe's car. I am gonna check with him and see how he did it to be exact.

Bottom line is I just want to get an idea of why it would mis on 7 with a colder plug. Could it be something besides cylinder pressure. When we switched wires,coils and plugs with 5 the mis stayed at 7.

 

7 lives a tough life...leak down test it.

 

I'd say you could have damaged the ringland on #7 or it could be a burnt valve. Do a leakdown test and you'll find out. I think its coincidental that it started missing with the colder plugs. I would check to see if your MAF sensor is clean also, a dirty MAF will make a lean condition at wot and #7 is usually the first to go.

 

Keep your head on a swivel and get home safe dude.

The numbers are low across the board and indicate either incorrect test procedure or a bad gauge. I've done compression testing on 3 LS1 engines and have never seen one lower than 185psi.

Nailing the throttle wide open during cranking is critical, you get whacky low readings w/ the blade closed.

I'd ask him to run a 100psi leakdown test on any cylinders < 190psi after he gets a good compression test done, so he can figure out where the leakage is happening. Hissing is easy to hear w/ a mechanic's stethoscope...intake valve leak hisses at the throttle body, ring leak hisses at the dipstick, exhaust valve leak hisses at the manifold.

Let us know what he finds.

 

Well weather or not you did the compression test right or not... you got 130 across the board but #7

Leak it down..

Betting on a cracked Ringland.

 

Thanks for all the input guys. What are the odds of the sleeve on 7 being screwed if it is a cracked ringland. The car runs fine and doesnt smoke. I just wonder if I should build up this 5.3 that I have sitting in the garage and wait for the ls1 to die? Or build the ls1?

 

Either way, they're the same block, I'd say if the motor is toast, it's 383 on the juice time.

Be safe man, I just got back a few months ago.

 

yeah id like to do a 383 but the funds wont allow it... after the coin I dropped on the nitrous system and tranny I dont have alot left to put in the car. Im looking at about 2k to 2.5 max to put another engine in it.