Compression and Timing

[zzAMBIENzz]

I feel like this is a really "noob" question, but:

Would I still show good compression if timing was off?

I have been chasing my tail for 2 days now, looking for the cause of the entire passenger side bank being dead. (cyl 2-4-6).

The Jeep starts and runs "decent" (as decent as you would expect running on only 3 cylinders). I have determined via dropping the coils that 2-4-6 are all dead.

Today I tested each coil, and all had great spark. I tested each injector socket with a noid light, and they all had pulse.

I also did a standard compression test, and all cylinders had 155-160 psi.

My initial thoughts on the problem was that timing was off. The engine has clearly been recently rebuilt, or at the least, has had head gaskets done. The compression test is throwing me off though. That seems like almost perfect compression. I would have expected either low or erratic numbers if the timing was off bad enough to have completely dead holes.

I am just looking for any information before I start stripping the front of the engine apart. The last thing I want is to tear it all the way down, only to find that wasn't the problem.

 

[Conundrum2006]

Well you need 100psi bare minimum to have enough compression to combust properly.
These engines are spec 175-225psi.

Far as I remember I don’t think these engines are interference engines. Maybe on the mistimed. But I would think it’d do something else you might notice.



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[Conundrum2006]

Sorry I have to post to reread the thread, I wanted to get the details of what it’s doing.

How did you test spark? Have you tried swapping sides (cyl 5 to 4)?




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[zzAMBIENzz]

Sorry I have to post to reread the thread, I wanted to get the details of what it’s doing.

How did you test spark? Have you tried swapping sides (cyl 5 to 4)?




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Yes, I swapped coils and plugs around, no change, still dead on 2-4-6.

Tested spark with a simple spark tester. Strong, bright blue spark jumping a fairly sizeable gap in the tester.

Even though it seems to me like everything points to timing, I keep running into things that keep turning me away from that being the issue. Without access to a 'scope, I can't definitively test for it.

With #1 at TDC on compression, both "V-6" markings on cam gears are straight up, and timing mark on crank pulley is right in line with the mark on the block.

Basic compression test seemed good, although if these engines are supposed to be as high as 225psi, then it was low, so that does maybe point back to timing. I was expecting "low" to be <100psi.

I am starting to think my options at this point are to just tear it down an visually check, or order a 'scope and wait. I was REALLY hoping to have this on the road this weekend, but the thought of tearing the entire engine down, only to find out that wasn't the problem is just eating away at me.

 

[Aceofspades]

Got fuel ?

 

[zzAMBIENzz]

Got fuel ?

I know have pulse to the injectors (noid light), but I have not been able to test for actual spray.

This was actually where I was at when left off working on it. One thing that was striking me as odd, is that the plugs are not fuel soaked ... to be honest, they don't even smell like fuel.

So that is one of the last things I can think to look for. Just trying to figure out how to test that the injectors are physically spraying with the equipment I have available.

 

[turblediesel]

I don't have the 3.7 but I've read about two braided grounds coming off the back of the heads to the firewall. Are yours still there and in good shape? Maybe one got left off during previous work.

 

[Conundrum2006]

Any codes? The injectors not firing should trigger a code


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