Compression and cylinder temps

[Jwhowell]

I have 1991 blazer all stock and I recently had dual straights put on. I’ve notice now I have a black sooty tailpipe on the drivers side and a nice clean one on the other. It recommended to me to check cylinder temps and compression. So these are my findings:
1 - 350 degrees & 170psi
2 - 414 degrees & 160psi
3 - 413 degrees & 165psi
4 - 383 degrees & 160psi
5 - 416 degrees & 170psi
6 - 374 degrees & 115psi
7 - 360 degrees & 170psi
8 - 261 degrees & 120psi

So I’m curious as to what the problem is and why no. 8 cylinder temp is so low. The other question is, are the two cylinder with the lower psi a concern being that they are so far off from the others. Any advice will help.

One more thing. I’m also chasing down a spark knock and everything I’ve been able to find is telling me either EGR or knock sensor. It is giving me a 43 code. Could anything else cause this issue?

thanks

 

[frankin5]

Since both cylinder 6 and 8 are passenger side and very low compression, i would be concerned. Could be a ring or valve problem. I would also check you timing and make sure it is set to 0 with the brown wire disconnected. This could be a reason for the code 43.

 

[rampage]

Also check the condition of your spark plugs, gives you good insight as to what’s going on in the cylinder.

 

[Jwhowell]

Number 6 plug was an ashy tan and the number 8 looked like it has fired at all. But it is sparking. I grounded it to see the if it sparked and it did. The spark was orange.

 

[Jwhowell]

Since both cylinder 6 and 8 are passenger side and very low compression, i would be concerned. Could be a ring or valve problem. I would also check you timing and make sure it is set to 0 with the brown wire disconnected. This could be a reason for the code 43.

Thinking about doing a leak down test.

 

[wasted wages]

6-8 are side by side....could possibly be a blown head gasket between the cylinders..

Is it running rough ??
hear any slight poping out of the exhaust?
headers or exhaust manifolds???

 

[dyeager535]

Forget the knock issue/code for now. The low compression in those two cylinders is a known issue.

Fix that, then if the code exists afterwards, deal with it. Low compression is not going to be caused by the injection system, therefore the code may be a symptom, and the cause is mechanical.

 

[Jwhowell]

6-8 are side by side....could possibly be a blown head gasket between the cylinders..

Is it running rough ??
hear any slight poping out of the exhaust?
headers or exhaust manifolds???

It’s running fine. No popping. There is a difference in how the exhaust sounds on that side. Doesn’t have the same deep tone as the drivers side.

 

[bigred88]

Thinking about doing a leak down test.

Do it. Low compression pretty much gives you 3 possibilities- leaky valves, leaky rings, or a blown head gasket. Two side by side that are similarly low, and none of the others low at all? Makes me think a blown head gasket too.

 

[Jwhowell]

Do it. Low compression pretty much gives you 3 possibilities- leaky valves, leaky rings, or a blown head gasket. Two side by side that are similarly low, and none of the others low at all? Makes me think a blown head gasket too.

I did the leak down test and found the intake valve on the no.8 cylinder was too tight and wouldn’t close all the way. So I loosened and the compression went up by about 30psi on no.6 and no.8 cylinder. Still lower than the rest. It brought them up to 140 and 135. The others are between 160 - 170psi. I’m thinking blown head gasket between cylinders.

 

[Wes Harden]

blown head gasket would explain the clean exhaust pipe. the pass side is being steam cleaned. and the coolant is lowering the temp and cleaning the plug in cyl 8

 

[bigred88]

I did the leak down test and found the intake valve on the no.8 cylinder was too tight and wouldn’t close all the way. So I loosened and the compression went up by about 30psi on no.6 and no.8 cylinder. Still lower than the rest. It brought them up to 140 and 135. The others are between 160 - 170psi. I’m thinking blown head gasket between cylinders.

If you only changed the intake valve on cylinder 8 and saw a difference in compression on BOTH cylinders, then there has to be a link between the two cylinders, likely a blown head gasket.

 

[Jwhowell]

blown head gasket would explain the clean exhaust pipe. the pass side is being steam cleaned. and the coolant is lowering the temp and cleaning the plug in cyl 8

During the leak down test I never heard any air coming out of the radiator.

 

[Wes Harden]

did you look for bubbles? what psi was leak down reg set at and what % of air was escaping. If you have access to block check I would do that test. coolant can leak from the intake to cyl 8, esp if aluminum manifold.

https://www.amazon.com/Block-Tester-BT-500-Combustion-Leak/dp/B06VVBSFTF

 

[imiceman44]

During the leak down test I never heard any air coming out of the radiator.

The head gasket doesn't always blow into the coolant side sometimes it is just between the cylinders like in this case

 

[wasted wages]

Called It !!!

 

[diesel4me]

I've seen a few engines that had the head gasket fail only in between two side by side cylinders and allowed the compression from one to be blown into the other,and they were ran long enough that way to have a groove cut in the block,like a cutting torch had been used!..the flame from combustion is hot enough to melt the block there I guess..weird thing was the cylinder head wasn't affected..