Low compression/misfire

[jake12tech]

Hey guys working on a 98 e300. Car has 395000 miles on it. Someone I know bought it off Craigslist and grossly overpaid for it considering I am not quite sure what’s wrong with it and if the engine is worn out losing compression which I doubt.

Brought it to me on a tow truck for me to look at it for him. It started up the first crank in 30 degree weather on the first glow cycle but it has a ROUGH idle. Like the engine rocks back and forth as if it’s misfiring, but it does shoot some white smoke out the exhaust with a “puff” sound. Not a whole lot of smoke like when you have a completely dead cylinder. Just enough to be noticeable.

However when you rev the engine to like 2 grand it smooths out doesn’t shake rock, or anything. Driving the car also it has VERY little power. Feels completely starved of fuel. Newer fuel filter.

So I know the step here would be to do a compression test, but I’m trying to save a friend of mine the money id charge to do that and then the price of parts to replace the injector o rings.

I don’t think it’s compression however because why would it go on the first crank dead cold start in 30 degrees? And if it was a dead cylinder it would not smooth out with revs.

Injectors are newer too as well. The PO said it was running quite well not long ago and this just started happening.

[Diseasel300]

A weak cylinder absolutely will smooth out with revs. Don't believe it? Crack an injector line and rev the engine up to 2 grand. You'll swear it was "smooth".

The "puff" noise in the exhaust is a clue...you probably have worn valve guides/seats causing a compression leak.

Compression and/or leakdown test should be in your future to diagnose further.

[jake12tech]

Originally Posted by Diseasel300

A weak cylinder absolutely will smooth out with revs. Don't believe it? Crack an injector line and rev the engine up to 2 grand. You'll swear it was "smooth".

The "puff" noise in the exhaust is a clue...you probably have worn valve guides/seats causing a compression leak.

Compression and/or leakdown test should be in your future to diagnose further.

I’ll try cracking the lines and see what happens. With worn compression doesn’t it surprise you it would go so easily on a first crank and fire up INSTANTLY? I mean I don’t doubt that it could be very well be compression, but it’s a little shocking it wasn’t hard to start.

Also I had this same issue with a 92 300D. I don’t remember if the exhaust was puffing or not, but the fuel filters were so nasty and the injectors and injection pump were filled with algae! The car idles exactly like this. https://m.youtube.com/watch?v=qJ6vQRImgOA

[jake12tech]

Really, this is one of those cars that has me really thinking.

Here’s more things I thought of?
What does cracking the injector lines prove?

It has 6 new (just about) glow plugs, but even with that in 30 degrees, if it had low compression wouldn’t the motor be hard to start? It went over the first crank and fired up immediately.

It has 6 new fuel injectors. I also realized I worked on this car not too long ago (local car) and remember driving it when it had about 10000 less miles on it. It definitely did not used to have this rough idle and low power problem.

[Diseasel300]

Originally Posted by jake12tech

With worn compression doesn’t it surprise you it would go so easily on a first crank and fire up INSTANTLY? I mean I don’t doubt that it could be very well be compression, but it’s a little shocking it wasn’t hard to start.

It only takes a single cylinder with a compression issue to affect the idle and low RPM smoothness. The "puffing" noise you mention in the exhaust is a clue. If the other cylinders are operating normally, you'd fully expect the engine to start normally, just with that one chugging cylinder. A 4 or 5 cylinder engine would be harder to start with a weak hole, but a 6 will still fire right up if the other cylinders are healthy and the glow plugs work.


Take the time to diagnose. Crack injector lines to isolate the problematic cylinder, then dig in further.

[Diseasel300]

Originally Posted by jake12tech

What does cracking the injector lines prove?

It has 6 new (just about) glow plugs, but even with that in 30 degrees, if it had low compression wouldn’t the motor be hard to start? It went over the first crank and fired up immediately.

It has 6 new fuel injectors. I also realized I worked on this car not too long ago (local car) and remember driving it when it had about 10000 less miles on it. It definitely did not used to have this rough idle and low power problem.

In my first reply, I was referring to a good engine. Crack a line to create a "miss". You'll notice it at idle, but rev the engine and it'll run smooth.


In troubleshooting the engine in question, cracking lines will find you the trouble cylinder.


New parts don't mean anything. If you have a mechanical issue that's developed since they were installed then you have an unrelated issue that needs a fresh eye.


You keep going on about low compression and starting performance. Get over it. You can have 5 healthy cylinders and 1 lousy one. If any of the 5 healthy ones light off, the car will start right up with a crap idle from the missing cylinder.

[jake12tech]

Originally Posted by Diseasel300

In my first reply, I was referring to a good engine. Crack a line to create a "miss". You'll notice it at idle, but rev the engine and it'll run smooth.


In troubleshooting the engine in question, cracking lines will find you the trouble cylinder.


New parts don't mean anything. If you have a mechanical issue that's developed since they were installed then you have an unrelated issue that needs a fresh eye.


You keep going on about low compression and starting performance. Get over it. You can have 5 healthy cylinders and 1 lousy one. If any of the 5 healthy ones light off, the car will start right up with a crap idle from the missing cylinder.

No it doesn’t at all. It points to me they were to trying to fix something. Probably has been an ongoing issue for awhile. I’ll just crack the lines and see what I get. If it points to a problematic cylinder I’ll go from there.

[jake12tech]

Nobody seemed interested in the thread but I’ll update it anyway.

The fuel injectors were recently replaced I believed I mentioned. I also heard an actual compression leak somewhere around the motor. Well it appears cylinder 3 was the culprit. They replaced injectors but didn’t replace any of The seals. I replaced the seals with a set I had laying around and it did RETURN power and resolve the idle. I didn’t do a leakdown test or compression test. Just backtracked recent work done. I talked to the owner and he said he installed injectors. I didn’t know it was as recent as in the last month.

Really goes to show a high maintained 606 is as reliable and good for its longevity as much as an old om617. My daily driver is always a testament as well considering it’s original besides a valve job and two timing chains. Not always the case with a high mileage motor, since I have had bad valves at 425k on these 606s. But majority of the time they’re fine.

[Father Of Giants]

Does it still puff white smoke or is everything fine now?
My 606 will put out the slightest wisp bone cold which is pretty normal.

The 603 on the other hand....I'm considering taking the head to metric motors in California......that thing is a smoker with 2 weak cylinders.

[jake12tech]

No more smoke except for 5 seconds from a cold start. Everything is good now and it’s got plenty of power. Your 603 sounds like the head is worn out. Does it use oil? Probably worn valves guides in your case.

[vwnate1]

WRONG ~ the GearHeads here who actually touch tools are always interested in follow ups , THANK YOU for this, I'd never heard of a pre used heat shield causing this so it's good to know .