i do not recall if you have retorqued your intake manifold or checked it for air leaks. also maybe check injector 5 intake oring. maybe it got nicked and is letting in air.

 

I have retorqued them and the shop that installed the harness retorqued them. I hooked a smoke machine to it and didn’t find any leaks. It’s acting the same as it did before replacing all injectors so I don’t think it’s an o-ring

 

you said you ran a compression test/leakdown test at the beginning of this post; do you recall what the compression readings (psi) were, as well as the leak down (%) readings?

 

Hi everyone,
I’m having a misfire issue on cylinder 5. Everything checks out fine except for the compression. I'm getting around 188 psi on four cylinders, but only about 115 psi on cylinder 5. After adding a bit of oil to the cylinder, the compression went up, so it’s most likely a problem with the piston, rings, or the cylinder wall.

Where I live, parts for the Hummer aren’t easy to find, and no one really wants to do a full rebuild on the 3.5 engine. I’ve been thinking about replacing the rings on just that cylinder and honing it. I also have another engine with crank bearing issues, so I could swap the piston from that one if needed.

Do you think it’s worth doing this kind of repair? I’m planning to drive it for maybe another 30,000 miles.

 

You should have started a NEW Thread with yur New and Different Question instead of tacking on the end of another member's thread.

Parts for a rebuild on the I5s are becoming difficult to find. Locate everything in hand before you start down that trail.

 

definitely, definitely, definitely get an air compressor and a leak down gauge and perform a leak down test on cylinder 5. It is entirely possible that cylinder #5’s valves are leaking which is causing the low compression. If it is the valves, you could just pull the cylinder head, have it rebuilt, then reinstall it. This is a much, much easier solution than performing an entire rebuild although you do need a certain specialty tool set to keep the engine in time while the head is off.

115 psi seems extremely low for it solely to be the piston/rings if the engine is still running fairly decent (no weird noises/etc.) It could potentially be a combination of the rings/valves/head gasket. Only way to find out for certain is a leak down test.

I am in the middle of a rebuild on my 07’s 3.7 engine and I can tell you this; it’s not been too fun. As Doc mentioned, parts are extremely hard to find (new balance shafts do not exist, you’ll have to resort to finding used ones which are scarce), crankshafts are only after market and are in the $700-1000 range if you need a new one, and you need several specialty tools to “properly” get the job done 100% right for even the tiniest things (balance shaft bearings for example require a $700-800 tool). It’s been a great learning experience, but also a bit of a PITA as well. And an expensive one at that.

If you do decide to go through with a rebuild, PM me and I can give you more info/let you borrow some of my tools if you wish, but my recommendation; perform a leak down test and check the cause of the low compression. If it is indeed only the pistons/rings that is causing the low compression, then I would just keep driving it, especially if it’s only roughly 30k miles.

If it is the pistons/rings/cylinder wall, you could also try a can of Engine Restore and see if that helps.

Let us know what happens!

 

I’ll definitely try a leak test, but I’m quite concerned about how much the compression increases when I add oil to the cylinder. The head on the other engine is like new, but I’ll still have it tested and reconditioned.

 

how much did the compression increase when you added oil to the cylinder? My H3 3.7 was averaging ~145-155 psi (very low) per cylinder due to intake valve leakage and a small amount of blow by to the pistons/rings. Adding a little oil to one cylinder made the compression increase to the 220-240 psi range, which is extremely too high (false reading) considering the new engine rating is 215 psi.

If you were getting somewhere in this range as well (a huge increase in psi simply by adding some oil to the cylinder) I wouldn’t think too much of it and would consider it a false reading. I would expect something in the realm of a 20-30 psi increase, but not a 70-80 psi increase like in my case.

A leak down test, will give you a much better idea of what’s going on, vice the standard compression test.

 

From 116 to 188psi.

 

THIS IS HOW TO HIJACK SOMEBODY ELSES THREAD. Get it?? Exactly why I said you should have started your own New Thread on your topic. Discontinue you jacking….. move along