Uh oh ....

[DeLorean03]

So I went for a drive today. The car was running absolutely beautifully. Then when I left a restaurant to head home, I noticed the car was stumbling bad, like the engine had a miss in it pretty badly. It was in all gears (auto trans) and drives. Drove with speeds up to 55mph, and the engine was missing the whole way home. Got it home, did the fuel pressure checks - all came out well. Looked at the plugs; they all were uniform with white/tan porcelain and clean electrodes.

Then I got to the compression test:

Cylinder 1: 105/115/120 (did it 3x)
Cylinder 2: 130
Cylinder 3: 130
Cylinder 4: 140
Cylinder 5: 130
Cylinder 6: 140

Obviously, the one I'm eyeballing is #1 ... and with x > 10% difference, I am not pleased.

So tell me, how bad is it? What do you all think? What are your thoughts after seeing those numbers (outside of the first word that came to my mind: DOOM).

NOTE: It just occurred to me, I did NOT have the throttle fully opened while doing this. I may need to re-do the tests tomorrow to make 100% sure.

[NightFlyer]

What you describe in terms of engine behavior sounds like an ignition problem. I'd start by checking or changing the plugs, wires, cap, and rotor.

Best of luck!

[Ron]

+1

Seems iffy for you to have a "noticeable" miss with those numbers. Definitely redo the check with the throttle AND plate open (W/O fuel pressure of course).
You might try pulling the plug wires one at a time while it is running to get another clue or two...

[DeLorean03]

Man, I am sucking at this engine compression test stuff....

It just also hit me, the engine was...."warm" in terms of being turned off for about two hours, but the aluminum of the engine was VERY tolerable - in other words - the engine was not "uncomfortably warm" .... and my understanding is the engine does need to be warm when performing a compression test for "less leakage" in the results.

So, are two people absolutely required for this? I did this by myself, and I'm kind of scratching my head as to why two people are needed.


Also, other questions:

1. Should ALL the spark plugs be pulled while doing each compression test, or is it ok to have all the spark plugs and plug wire boot ends installed while testing any one cylinder?

2. If I'm by myself, could I just hold down the accelerator while cranking the engine? Wouldn't holding the accelerator pedal down fully accomplish both the throttle plates being wide open and the air-flow sensor plate being depressed fully?

3. I have now heard of doing both a wet and dry test on each cylinder. The wet test is done by putting in "30 weight oil" into a cylinder. My question - is 30 weight oil simply available at any auto store? I did a search and got some results such as this, but I just wanted to ask and make sure from everyone here:

30 Weight oil...?

I sure do hope it's just ignition related. Having a miss while driving at 55 mph made me a bit uneasy to say the least.

[Silverbullet]

The 30 weight oil is to try to guess if it is rings of a valve... the oil will seal bad rings for the test....

[Ron]

It doesn't matter if you check the compression on one cylinder at a time or not.
To find a miss due to compression, it don't really matter hot or cold.
The second person is so that one can ensure that there is no restriction of air getting into the cylinder(s)....Idealy, you want to hold the plate open, ie do not make the air flow open it.
No reason to use oil for this situation, it will only aim you to why the compression is down (if it actually is). -- You can use the same oil you run in the engine W/O any problems (A lot of Ds running 10W30 out there. If a store has oil, they have a 30 weight of some kind ;-)

[Jim D]

I also agree that it sounds like an electrical ignition issue. Just a quick simple test is to wait until it's dark and start the engine, accelerate and look for arcing around the plug wires. A plug wire may have come lose or have a short, I can't see compression being an issue based on how you described it. Good luck!

[Kenny_Z]

When I did a compression test on my Monte the engine was cold, fuel and spark were shut off, and I used a remote starter trigger. You turn the engine over until your tester peaks a couple of times at the same range. It should only take a few revolutions. I'd assume on the D you could just take the W pipe off to make sure enough air was making it into the engine. I never had that issue with the Monte.

On a compression test you don't want more than a 15 percent variance between the highest and lowest reading. You can use a little oil in the lowest cylinder to see if the valves or the rings are an issue. If the reading is the same your valves are an issue and if the reading is higher your rings are the problem. Your compression is fine. The culprit is something else.

[Dangermouse]

Also check that an injector hasn't popped out. I had that happen once, due to weak spring clips I believe. Didn't pop all the way out, but enough to throw the firing off.

[David T]

Compression isn't the first thing that would come to mind for a miss. You cannot rule out a fuel problem either, like a misbehaving injector. As for the low compression, you may just need a valve adjustment since you say all of the plugs looked good, it is probably not rings. I agree, for it to come on suddenly it is most likely some kind of electrical/ignition problem.
David Teitelbaum