Dodge Grand Caravan 1999 - 3.8L low compression on one cylinder

[MaximumMini]

Hi, first post here.

I have a newly acquired 1999 Dodge Grand Caravan 3.8L

If I didn't know cars and engines well, I would probably keep driving this vehicle since it pulls hard and feels darn near normal when driving and idling.
But there is always someone who knows more or has had similar experience.
Problem is, it is throwing an occasional P0302 (misfire cyl 2) code and bucks very slightly at idle when in gear.

removed rocker arm, no obvious damage to rocker pedestals, springs or rocker arms. looks good.
Did a compression test and got 60psi (very low). Adjacent cylinder 190psi (very good)
Did a leak down test and it was inconclusive. No clear indication of where the leak was. But it was large (85%)
I was really dissappointed in the leak down test. I had expected clear evidence of the source.
I did notice a small amount of air coming out of the dipstick tube when I removed the dipstick. But it seemed like too little to be significant.

Poured a bit of oil into the suspect cylinder and compression went from 60psi to 100psi.
Would that be enough for you to conclude a problem with the rings?
Tomorrow I'm going to look inside the cylinder with a borescope and see if I see cylinder wall damage. If I do, game over. engine comes out for a rebuild.
If no damage, I'll probably take the head off and inspect the head and head gasket.

I've started adding Marvel Mystery Oil to the cylinder yesterday to let it soak a few days and see if the rings might be stuck by carbon buildup from cheap gas etc. I've read many accounts of it unsticking stuck rings and restoring compression.

Any suggestions or comments welcomed.
Thanks

 

[Lubener]

I would bet you have a burned or sticking valve. However, from your oil test, the rings are not in the greatest shape but still 100 psi is not very good. You got nothing to lose with a piston soak. All cylinders must be within 20% from highest to lowest for the engine to run smoothly.. You have to start by removing the head anyway and you should check the valves at that time..

 

[MaximumMini]

I would bet you have a burned or sticking valve. However, from your oil test, the rings are not in the greatest shape but still 100 psi is not very good. You got nothing to lose with a piston soak. All cylinders must be within 20% from highest to lowest for the engine to run smoothly.. You have to start by removing the head anyway and you should check the valves at that time..

Sounds logical to me.

What's crazy is that the vehicle drives fantastic. Feels smooth and very good power under load. Almost as if whatever the problem is significantly resolves itself under the pressure of load. There is no way I know of to check the check compression while the engine is under load. A leak down test doesn't apply anywhere near the pressure of combustion.

Odd case.

I'm preparing to remove the head in the next week.

 

[SwampSurvivor]

My Grand Marquis was like that when I bought it.

I wouldn't have done anything if I didn't need the check engine light off for NYS inspection.

 

[MaximumMini]

My Grand Marquis was like that when I bought it.

I wouldn't have done anything if I didn't need the check engine light off for NYS inspection.

Ya know, that same slight "surging" at idle in gear can also be caused by a torque converter.
What did you do?

 

[SwampSurvivor]

Ya know, that same slight "surging" at idle in gear can also be caused by a torque converter.
What did you do?

I tried a few things to get it passed inspection. Bumping up the throttle body idle screw ... worked for a few minutes until the IAC compensated. If I ran the AC/Defrost all the time ... it would bump up the idle enough that it wouldn't misfire in gear. Part of why it was setting a light was the civilian cars idle at 490 RPM in gear. The 600 RPM idle with AC on or while warming up was fine.

There was a definite gallop while cranking. I had a shop do a leakdown test on it and it was "yes" coming out of both valves. After pricing what it would run me to get a valve job (assuming rings were still good), head trued up, head gasket job .... doing all of the work except machine shop work myself - it was cheaper to buy a used engine with a warranty for $500 and swap that in.

 

[MaximumMini]

Almost got the head off today.
It should be out tomorrow.

What is causing this low compression? The suspense is killin me.

 

[MaximumMini]

Oh, before I took it apart I peeked inside the cylinder with a borescope and didn't see any damage. Cylinder walls looked ok.
Could still be a bad head gasket.

Thing is, it has 157,000 miles and once the intake and heads are off it's just a little bit more work to get the whole motor out for a nice fresh rebuild. decisions decisions.

 

[Lubener]

My Grand Marquis was like that when I bought it.

I wouldn't have done anything if I didn't need the check engine light off for NYS inspection.

What did you find?

 

[SwampSurvivor]

What did you find?

Very low compression on cylinder 5. Since these 2 valve 4.6s are generally pretty reliable engines, I figured a junkyard engine for less than a valve job would be a safe bet. And it was.

Everything looked good but the valves weren't sealing great for some reason.

 

[MaximumMini]

OK, head is off....the verdict is in......
Number 2 was the bad cylinder
Can you see why this cylinder had a 85% leak condition?

cylinder walls smooth no scars, gouges, marks or cracks

(I can't see it either)

 

[Exhaustgases]

Yeah the spark plug is missing.

Is that etching?

 

[MaximumMini]

Yeah the spark plug is missing.

Is that etching?

So....the head being off won't cause low compression ? <scratches head> haha

etching? tell me more.

 

[Exhaustgases]

Fingers versus photo's and close up eyes/ hands are just way better than a picture.

 

[MaximumMini]

Fingers versus photo's and close up eyes/ hands are just way better than a picture.

Yep. I'll buy you a bus ticket. When ya coming?

 

[meep]

Exh valve is a different color - but I can’t see anything visually.

We know there is a ring sealing problem since oil changed the compression numbers. We also know there’s a valve or other top end problem since the with-oil numbers were too low still. You said this was a 3.8? Thats a bored and stroked 3.3 (lovely family of motors IMO) but the 3.8 can get a little sloppy with more side loads on the piston, and arguably thinner cylinder walls.

You might be able to drop the pan and re-ring the cylinder by just poking them all out the top. You could have the head machined and lap the valves. It would save you the full block-out effort. Is there a ridge at the top of the cylinders?

Also would be a good idea to get a micrometer and simply measure the cylinders and multiple points to see if they are consistent, straight, etc.

 

[MaximumMini]

Exh valve is a different color - but I can’t see anything visually.

We know there is a ring sealing problem since oil changed the compression numbers. We also know there’s a valve or other top end problem since the with-oil numbers were too low still. You said this was a 3.8? Thats a bored and stroked 3.3 (lovely family of motors IMO) but the 3.8 can get a little sloppy with more side loads on the piston, and arguably thinner cylinder walls.

You might be able to drop the pan and re-ring the cylinder by just poking them all out the top. You could have the head machined and lap the valves. It would save you the full block-out effort. Is there a ridge at the top of the cylinders?

Also would be a good idea to get a micrometer and simply measure the cylinders and multiple points to see if they are consistent, straight, etc.

Thanks Meep. Good stuff there.
Everything you posted sounds on point.

Small ridge. But I have a reamer so I could take care of that.
My hope is to find (and eliminate) the cause if I can. Why this one cylinder? Oiling issue? Head problem?
Considering the severity of the leak (85%), I was expecting a head gasket issue, burned valve or something a bit more obvious.

I checked the cam by measuring the lift of each lifter one by one. They were all within one or two thou of each other. No cam issues.
Tomorrow I'll pull that piston out and check the rings. The cylinder walls look great.

I'm also going to pull the valves on that cylinder out to see what the seats, guides and underside of the valve looks like.
Bugs me that as of yet I haven't nailed down the culprit for such a large compression leak.

 

[Marcozi]

I think you answered you question in the original post: you put oil into the cylinder and compression went up. I'm betting you've got either a bad compression ring or the rings are gummed up.

 

[thastinger]

Looks like the head gasket is blown to the bolt hole, probably overheated then traded in when the issue from overheating revealed themselves.

 

[MaximumMini]

I think you answered you question in the original post: you put oil into the cylinder and compression went up. I'm betting you've got either a bad compression ring or the rings are gummed up.

It went from 60psi to 100psi (give or take 5 psi)

Is that a significant enough change to point to just the rings in your opinion?