Update to smoking Isuzu-- dead cylinder in Isuzu!

Well, I just did a compression test on my new 92 Isuzu Rodeo with the GM 3.1L engine and there was oil on the #1 plug and the compression on that cylinder was about 20 PSI dry and 30 with a squirt of oil. Yikes! The rest of them are all at 145-150, except for the #4 which is only at about 120.

Anyways, the truck runs suprisingly well on 5-cylinders (give or take), so I’m wondering if it’s possible to just disable the #1 cylinder to get the thing to quit smoking? I’m thinking if I take out the pushrods, that cylinder’s got such low compression anyways that it’ll just move back and forth and not hurt anything. One problem would be that it’s a TBI engine with an injector for each side, so the #3 and #5 cylinders will get three cylinders worth of gas-- any ideas on how to compensate for that? Maybe if I take out #4 too so the engine could compensate with both injectors? Am I nuts?

I’ve talked to my professional mechanic and he thinks it’d be a gamble to just replace the rings on that one cylinder, but the truck isn’t really worth a rebuilt engine and because the starter is on the other side from the GM version of this engine, it’s pretty hard to find used at a reasonable price.

A compression check won’t indicate if the compression loss is from the valves or the piston rings. A vacuum test and leak-down test will do that.
As far as disabling the cylinder, the CPS (crankshaft position sensor) may see that as a misfire on each crankshaft revolution.
If you find that it is a piston ring, and you’re doing the work, I can’t see where it’d hurt to hone #1 cylinder and put new rings in it.

With compression that low you might consider a valve problem also. Bring that cylinder up to TDC on the compression stroke and apply some low pressure air to it. If a valve is also contributing to such extremely low compression you should hear air hissing either out the exhaust or the intake; or both.

While it’s always a risk, I have seen partial fixes done with some success and even did this once myself many years ago when some comedian sabotaged a small block Chevy by dropping a nut into the carburetor. This of course wound up being ingested and cracking a piston top. It’s not recommended as a proper repair but in a pinch it will work and work fine.

I don’t think this engine has a CPS-- it’s a TBI with a distributor. It’s a really simple setup-- it’s probably the least complicated looking engine compartment I’ve seen this side of 1973. I’m suprised that with the completely dead cylinder that nothing triggered the check engine light, since the thing must be pumping a lot of unburnt fuel through there.

The thinking on it being a gamble to fix the one cylinder is that this is a 4wd truck and it’s quite a lot of work to get the oil pan off and, since there’s no convienient nut explaination, there’s no reason to think that the other ones are on the way out, especially #4.

It seems to me that the valve issue wouldn’t lead to as much oil winding up in the cylinder and generate some compression, but it is certainly worth a try.

You can use the forum Search function on the red banner, above. In Keywords, place “low compression”. The first post will be by Karc5a, titled, “2003 Jeep Liberty 3.7 miss-fire”. His engine had a #1 cylinder compression of 30 psi. He found that it had bent valve spring and bent valve. He repaired that, and then everything was sweetness and lightness. See for your self.

Hmm… well maybe that is worth a more serious look then. Thanks!

Okay, so I took it to my local old-timer mechanic this time for a leak-down test, but he said just on my description and on the compression test results that he was reasonably sure it was a burnt or otherwise damaged valve. This sounds a lot better to me than a broken ring and is probably what I would think if all I had to go on was the compression test, but I can’t think where the oil’s coming from. When a valve burns, does the seal usually go with it?

Also, for that matter, if the cylinder’s got no compression presumably there’s no combustion going on in there, but then how is the oil getting burned exactly? Is it just going through the exhaust and burning from heat or getting burned up in the catalytic converter?

If the valve is burned, then the seal might also be baked and ruined which easily allows oil into the cylinder. I should also point out that no compression does not mean no combustion; fuel still gets inhaled and ignited by the spark plug. It just means the explosion cannot be contained to produce any meaningful amount of power to the engine.

It sounds like a valve job will fix the problem. This is about the same as a head gasket repair with a cylinder head reconditioning. But, if you feel the truck is not worth that, disabling the intake valve on cylinder #4, and removing the spark plug wire will kill that cylinder for good. Leave the exhaust valve working. The TBI will adjust the fuel mix based on air load through the engine, so no harm there. And a '92 ODB-I system will probably not recognize a spark plug is not firing. These systems were fairly stupid compared to ODB-II.

Is the TBI smart enough to do that? There’s two fuel injectors and I think one feeds one side and the other feeds the other one, but there’s only one oxygen sensor and I don’t think it can control the two injectors indiviually so it seems like it’ll be constantly cycling between too lean and too rich. But anyways, the truck’s not worth a ring job, but it is worth a valve job, especially since my old timer quoted me 700 bucks which seems pretty good to me.

This might warrant another thread, but I’ve noticed you can get a rebuilt head for this engine for only about $150 and I’m guessing that’s about or only marginally more than what the labor charge for my mechanic actually regrinding the valves is-- would it be better to try and get him to just put one of those in since they come with new valves, springs, seals etc? I guess it kind of depends on what his “valve job” actually entails-- I get the impression that it’s sort of like a tune-up in that there’s no set definition.

If you can get a good quality rebuilt head for $150, I can’t imagine getting yours rebuilt would be much less, and it would probably be more, for the same quality work. I’d go for the $150 rebuilt head.