Potential Dead Cylinder - Double Check Me

[BoostedE21]

Hi all,

I purchased my '91 a couple weeks ago, and am trying to diagnose rough running at all RPM. The car starts reliably, but takes an extra second or so to 'catch', and then runs as if it's on 5 cylinders. This happens at all RPM. It's not intermittent, it's consistent. Thus far, I have been doing basic baselining work like new spark plugs, wires, injectors, and O2 sensor. I also cleaned the MAF sensor.

Today, I pulled the cat, thinking it was clogged, and noticed it had been all pushed through, so no, definitely not clogged.

while the cat was out, I pulled the fuel pump relay and cranked it for a couple seconds. Below is the result (turn on the audio). It sounds to me like one cylinder might not have full compression....you can almost hear a noise like air rushing (not sure if it's sucking or blowing).

 

[LG57]

Time for a compression check. Borrow a gauge from your auto parts store,

 

[92exp4x4]

I agree, these engines are known for cracking the cylinder head between the valves. The early 90TM casting heads we're more prone to it. Your truck will have this casting if the engine is original.

I bet she got hot in the near past. You're probably gonna find a dead cylinder.

 

[BoostedE21]

I agree, these engines are known for cracking the cylinder head between the valves. The early 90TM casting heads we're more prone to it. Your truck will have this casting if the engine is original.

I bet she got hot in the near past. You're probably gonna find a dead cylinder.

Is that a repairable issue, or am I looking at buying reman heads on RockAuto or EBay?

 

[410Fortune]

it depends on the extent of the damage.......
You need to do a simple compression test, and then a leak down test

Pulling the 6 spark plugs and looking at them can point you to the troubled cylinder
Also check the exterior of the block with a flashlight to see if you can spot where coolant maybe leaking from the heads

 

[BoostedE21]

it depends on the extent of the damage.......
You need to do a simple compression test, and then a leak down test

Pulling the 6 spark plugs and looking at them can point you to the troubled cylinder
Also check the exterior of the block with a flashlight to see if you can spot where coolant maybe leaking from the heads

Got it. I don't see any evidence of coolant leaks - nothing in the oil, nothing burning, no leaks on the ground. So that's a start....but yes, I'll do a compression test and leakdown.

 

[96eb96]

Is that a repairable issue, or am I looking at buying reman heads on RockAuto or EBay?

K-seal solved this problem for me (and others) 100%, YMMV. It is one product that does not do "what it is not supposed to do." First verify that you indeed have that common issue. Mine runs very good now.

 

[BoostedE21]

One other question for y'all - I'm doing a compression check on Sunday to determine exactly which cylinder might have the issue. If I do find a burnt valve, can I rebuild just that one head (surfacing, replace the dead valve and seat) and reinstall, or do I need to do both at the same time?

This particular explorer is not my primary vehicle, and I'm concerned about getting over invested in a "weekend car" if it doesn't make much difference.

 

[LG57]

Actually you can do the work to only one head. The thing is you'll have it torn down to the point you might as well do both. You will find yourself nickle and dimeing? and be right back in there doing that other head shortly.

 

[BoostedE21]

UPDATE:

Did the compression test today.

Passenger side bank is 130 psi on all three.
Driver side is 110-90-0 psi.

Question is, what would cause a cascading loss of compression across three cylinders? Head gasket? Cracked head?

I was originally thinking burnt valve, but maybe not since this gets progressively worse from front to back.

 

[410Fortune]

the rear cylinders suffer first on these engines, they have the most issue with getting oil pressure
At this point it could be head gasket, cracked head, valve seals, stuck valve, bad lifter, bad rings, so many things
When you add a cap full of oil to the cylinder does compression come up from those numbers?
Leak down test can help determine what is next, but with those numbers I would just pull the head
Yes you can do just one side and replace just the parts that are found to be bad
MANY ohv 4.0 go down the road with just one new lifter, one new head, etc etc.... we are all on a budget and when you DIY the nice thing is you get to decide what parts go where and how much time and $$$ to spend.

 

[Roadrunner777]

Yeah, you're going to have to turn some wrenches to know for sure, and you're going to have to pull that head to fix whatever it is. Previous history would be interesting, but academic. Perhaps the good head was rebuilt at some point, perhaps the bad side overheated at some point.

I appreciate your position on how for to go for something that is not a primary vehicle... or even a primary vehicle. When I do something, say brake pads, it is always a mental debate... the disks are rusty and worn, but serviceable, well, I'll replace them just in case. Well, the calipers, since I have them off anyway... might as well do the rubber brake line... and so it goes until a $25 2 hour task turns into a $150 all day task. (And on the Explorer, you really begin to start thinking about rebuilding hubs at that point.)

But I think I would ask you to think about your long term plans for the vehicle. If this is a hobby thing, maybe you should cut your losses. But, if you see yourself driving it for years to come... I mean, lets say you drop (I guess) $800 for new heads, rocker arm assemblies, pushrods, and the misc goodies to really get the engine top end right. You probably won't recover that $800 if you go to sell it. So, this isn't a financial question, it's more about whether you feel a connection to it personally.

And if you do feel that connection, then don't do this half-way. I would plan on new heads, rocker arm assemblies, pushrods, and really do a job so you don't have to come back to this next year.... unless you really have a love for turning wrenches and fixing stuff, which I would understand... but if you like turning wrenches, there will be plenty of adventures further down the road.

And I guess just because I feel like typing... you might ask yourself: Ok, if the engine is running perfectly, what else do I need to address to really be satisfied? Is there rust? Any other unrelated issues? If it's an automatic, do you have a sense of confidence that the tranny is in good shape? The auto trans on the Explorer is not exactly a weakness, but it's something that does tend to fail when pushed to it's upper limits, particularly if it doesn't have a cooler or hasn't been maintained.

Anyway... LOL, good luck!

 

[DrewDude320i]

Got the heads pulled today - see if you can spot the cause of the compression loss!

fortunately the bores look great, so the chunk of valve must have just left through the exhaust.

off to be rebuilt!

edit: apparently my iPad is signed into an old account I had on this site, but it’s me, OP!

 

[410Fortune]

congratulations!

That is a new one to me, I have never seen a valve missing a piece
Any cracks in the areas between the valves? (Hard to see on the cylinder at bottom)

 

[BoostedE21]

congratulations!

That is a new one to me, I have never seen a valve missing a piece
Any cracks in the areas between the valves? (Hard to see on the cylinder at bottom)

Yes indeed, crack between the valves on the rearmost cylinder (top in the picture). Also had a head gasket failure between all three cylinders, which explains the compression leak.

The machine shop ordered a new casting, and it'll be all good by next tuesday!

On a semi-related note, one of my exhaust bolts broke off when removing the manifold - where can I find a replacement? Any special material?