Author Topic: sporadic low compression in one cylinder (Read 6496 times)

ianders · « **on:** March 18, 2009, 03:39:15 pm »

Hi everyone,

I have been reading this forum extensively over the past few weeks as I stumble through my new project, but this is my first post. I just had to put my last car ('82 Mercedes 300D) out of her misery due to rustjavascript:emoticon(':('). I love diesel and am happy to have acquired a '91 Jetta EcoDiesel. I am really excited about the gas milage and having a stick shift again!

The body is in great shape- very little rust, but I knew the engine would need some work. It would start fine and run great for a while, but as the engine reached temperature, a sudden change would happen and it would loose power, smoke and generally run rough. Sometimes it runs great for a while, sometimes it doesn't. The previous owner said that these problems happened after he had it serviced by a local mechanic who replaced the timing belt and a valve spring. I don't have much faith in the work this mechanic does- when he couldn't fix the problem he tried to make up for it by spraypaining the valve cover and rims with silver spray paint.(it was a nice gesture I guess- the rims look okay) He is no longer in business.

I have already done a lot of work on the front suspension and brakes, and have finally moved onto the engine. I just received a full filter change kit, all new belts, new injector nozzles and heat baffles. Here is what I have done to the engine so far:

checked fuel system for leaks and water
checked air filter
checked that the cold start lever works properly
ran a can of diesel purge
verified that the timing it set and the belt tensioned properly (have not checked start of injection timing yet though)
cracked the injector lines individually to isolate the problem
tested the compression

This is what I found:

After cracking the lines, I could say that the problem in in cylinder two.
the compression test verified this (engine warm)

1- 480 psi
2- 50-100 psi (not very consistent numbers)
3- 480 psi
4- 460 psi

The injector nozzles seemed to be pitted an unevenly worn in the center.
Injector #4 was very hard to remove which I found was caused by the soot packed all around it.

At this point I am at a loss of what to do next. I suspect that the problem is in the valves for cylinder two- possible something to do with the work the mechanic did. I pulled the valve cover and everything appears to be fine up there, but i guess there is not much to see without removing the head.

I am really confused as to why this problem is so sporadic. Tomorrow morning I am going to check the compression again when the engine cools down to see if that cylinder has compression, since the car runs great when it is cold.

I would love any help you all can offer. I don't have a huge amount of experience working on engines yet, so dont feel bad about mentioning the obvious.

Thanks in advance for the help!

burn_your_money · « **Reply #1 on:** March 18, 2009, 04:10:44 pm »

Quote from: "ianders"

The injector nozzles seemed to be pitted an unevenly worn in the center.

That's completely normal for nozzles that are, or have exceeded their life expectancy. I wouldn't recommend swapping nozzles without setting the opening pressures. They will be all over the map.

50-100 psi? I think you should do a leak down test

Smokey Eddy · « **Reply #2 on:** March 18, 2009, 07:27:10 pm »

50psi sounds to me like a valve isn't seating properly (as in not at all...)... that's REALLY low...

frightfully low....

50psi? really?

did anything hard go into the intake at some point? like a nut? :roll:

Dakotakid · « **Reply #3 on:** March 18, 2009, 08:08:01 pm »

My vote is that the "mechanic" who installed a timing belt had it out of time and rolled the engine over with the starter (hard--multiple attempts) and bent a valve on that cylinder.
He may have grunted the crank over with a long wrench (or tried).

ianders · « **Reply #4 on:** March 18, 2009, 08:59:39 pm »

My best guess is that it is a valve issue also, but I don't understand how the engine could run fine for a while, and then switch over so suddenly. Has anyone experienced anything like this?

Tomorrow I am going to check the compression cold, and then if i can find or make a leakdown tester, i will try that too.

ianders · « **Reply #5 on:** March 19, 2009, 08:45:43 am »

Still No compression this morning. I did some sort of variation on the leak-down test. I hooked air up to my compression tester adapter, and with the cylinder at TDC, could hear the air rushing out into the exhaust manifold. I guess that settles it.

About to remove the head.

subsonic · « **Reply #6 on:** March 19, 2009, 10:05:00 am »

Which cyl had the valve spring replaced? Do you know if it was an intake or exhaust? Did you get any info on why it was replaced?

Fisher · « **Reply #7 on:** March 19, 2009, 10:21:45 am »

Pull the Valve Cover and check the Lifters, they should turn by hand with the Valves closed. Double check which Valve Spring was changed.

I recently started a new Engine that I built for a 91 Jetta, zero compression in one Cylinder. Ended up being a Lifter was sticking and not allowing an Exhaust Valve to close. First time I ever saw one stick pumped up.

Put new Lifter in, replaced Cam, ran perfectly.

zukgod1 · « **Reply #8 on:** March 19, 2009, 10:48:46 am »

Sure sounds like your going the right way with it.

I agree with the above posts and your findings. Exhaust valve.

The good thing is that's easy to fix once you get the head off, at least you don't have to pull pistons etc.

subsonic · « **Reply #9 on:** March 19, 2009, 11:02:37 am »

x2

It will also give you a good visual of the condition of the head and pistons. You will be able to see if there was piston/ valve contact etc.

I may have missed it above, but how many miles on this engine?

What did you do with the old merc diesel engine?

You live in Woostah :lol:

ianders · « **Reply #10 on:** March 19, 2009, 02:45:43 pm »

The engine has about 200k on it.

The head is off!

So far, the pistons and valves look fine. There is absolutely no sign of any collisions. The valve even looks like it is seated, or at least it is not obviously proud of its seat. All of the lifters look good and spin freely. The only thing odd that I noticed was that one of the lifters on the first cylinder had about 2mm of spring to it when i pushed on it. The others were all firm. Could this be because the engine had been off for a day and the oil weeped out of the hydraulic lifter?

Finally, there is a small crack between the two valves in cylinder number two. Maybe this is the problem? I will try to post a picture later tonight.

subsonic- i stripped the parts i needed from the benz then sent the rest off to the car graveyard

Rabbit TD · « **Reply #11 on:** March 19, 2009, 07:52:35 pm »

I have a feeling it's the lifter on that #2 exhaust valve myself, this engine does have the hydraulic lifters doesn't it? If not check and see what the clearance is when it's cold. It could get to the point it holds the valve open after things warm up. I'm pretty sure they are hydraulic though. I would find a known good lifter and make sure the valve is free in the guide at the same time. Most of these engines have those cracks in that place and most of them don't cause any problems and even then I doubt it would bleed off that much compression that fast but a valve would and you can hear it in the manfold as well,from what you said. I think I would just put a new valve in at the same time and hand lapp it as well and then all shhould be good in that hole, assming the spring isn't broken as well.

ianders · « **Reply #12 on:** March 21, 2009, 10:44:22 am »

I finally had time to take apart the head this morning.

Sure enough, it was a bent exhaust valve on cylinder#2. I actually had to drive it out of its guide with a drift punch. Right now I am trying to decide if i should rebuild the entire head, or just fix up that cylinder.

What tends to be the going rate to have a head checked and guides replaced by a machine shop?

Dakotakid · « **Reply #13 on:** March 21, 2009, 05:26:10 pm »

Well for starters, please take the time to find a machine shop that regularly works on these engines (or at least used to). The head can be destroyed by pressing in the guides with too much psi. Then, there is the issue of whether or not they will give you any guarantee for their work if you go back in with used valves.
There needs to be a pressure check performed after the guides go in. Head work has complexities. My local shop is fussy....but my N/A's tend to go down the road pretty well.
My shop likes.....aahhh....cash (greens) for their work and it seems like $35 or so for an answer to your specific question. Wonder why they like cash.......

?

Rabbit TD · « **Reply #14 on:** March 21, 2009, 05:40:48 pm »

Glad you found that it was a valve, I wouldn't be afraid to just get a new valve and as long as the guide isn't binding on the new one and slides easily without excssive clearance, just clean it all up good and run it if there was no other damage to the lifter, use new valve seal of course. I'm not sure about those hydraulic lifters though as to how they can be damaged. I doubt if there is realy anything other than that really wrong with it though. All you're out is a head gasket if you want to give it a shot though and if it were me from the way you've described how it ran before I definately would.