I need to perform a compression test on my 1989 740. The plugs in cylinder 2 and 3 are seized.

Do they need to come out to check the compression in cylinders 1 and 4?

 

A proper compression test is performed with all plugs removed. Why are you performing the test? My guess is that your car is overheating or not running properly and the odds are that the seized plugs are related to the problem

 

A year ago i replaced the cap, rotor, wires and the 2 plugs that came out smoothly to fix a stalling no start issue that only happened in wet weather.

Last October my child drove the car just over 6 miles with the cooling fan and water pump belt in the trunk of the car. It overheated.

The other day my child had issues getting it started. It has spark. The fuel pump can be heard when the key is turned. It cranks. It acts like it really wants to start. I think it has no compression.

I've been pblasting the stuck plugs and understand that all 4 should be removed...but thought that I might be able to check cylinder 4 whilst cylinder 2 still had a plug in it.

 

Hey at least you are soaking em in penetrant.

hopefully the headgasket isnt popped. Steam out tailpipe?

I doubt compression is the issue. You would be able to tell if low compression=fast cranking.

 

Does the car still run? Put all the plugs back in, start it and run it to get the plugs hot--much better chance of getting them unseized this way! Don't overheat it anymore, of course!

 

I got them all out. Did a dry compression test.

Cylinder 1 had 80 psi

Cylinder 2 had 120 psi

Cylinder 3 had 135 psi

Cylinder 4 had 120 psi

I haven't looked for the specs yet but i think the head gasket is blown around cylinder 1. The gasket is 10 years and 100,000 miles old.

 

You want them to be above 120 and within around 10lbs of each other. Something is definitely going on with the #1 cylinder. Time to pull the head and find out what the problem is

 

no it's time to add a oil containter cap full of oil to that cylinder and do a WET test to see if compression goes up=bad rings. We had a bad cylinder under 100psi on a junkyard engine recently. Dumped ATF in the cylinder and left it overnight, blasted it out via cranking in the morning and it broke the rings free and we got compression back up over 120psi!

If it fails a wet test, and the ATF test doesnt help, time for a leakdown test, but that will never happen around here....

 

So i added a capful of oil to cylinder 1, then a bit more, cranked it over with gas pedal to the floor (which i had neglected before) and the psi increased to 180.

I decided to let it sit for a day and do the test again from the start.

I do not have compressed air and am unable to do a leak down test.

Whilst I was messin with it i noticed the crank position sensor has badly frayed wiring...but taking the cps from my 1992 940 did not help.

 

I don't know about adding much oil to the cylinder to do the test. If you add too much, you definitely would increase your compression as the oil will decrease the compressible volume of the cylinder. Back when I ran old air cooled VWs, they said just take an oil can and just squirt a bit in there, run the starter for 5-10 revolutions to splash it around and then re-test. If the compression increased, then it meant that you had worn rings. To unstick a stuck ring, they recommended running a small bottle of diesel through the carb. I think that did the same as seafoam.

 

Sounds like bad or stuck rings. Do some google research on compression tests. Has the car been sitting for a while? If so, they could just be stuck and the ATF trick might could work.

 

there's a few good vids on Youtube on how to do the compression and leak down tests. Search "Robert DIY" to begin - he's legend on the 850 board :-) To note the wet test doesn't take much oil, 1-2 tablespoons should be enough to coat the cylinder walls. Add, crank, retest. You can also try doing a wet test on all cylinders using the same amount of oil to get a sense of the differences. No harm in squirting some Seafoam in the carb (or ATF in the cylinder) if you're looking at opening up the engine as the next step...

 

I put some new plugs in and got the car to start and run very roughly for just a moment. The oil on the dipstick looks like chocolate milk, the oil cap looks fine but whats sitting under the valve cover also looks like chocolate milk. Ive changed the oil every 5000 miles for the 2years and 25000 I've owned it.

I watched/read some more on compression tests and now have a better understanding of just how much oil to use and depressing the gas pedal.

I do not have access to compressed air to do a leak down test.

 

You can just remove the fuel pump relay instead of stepping on the gas pedal, same effect. Anyway; a milkshake in the oil pan is a new head gasket at a minimum.

 

Milkshake = time to pull the head

 

I decided to go ahead and pull the head. I'll then send it to a shop. I've never done that before....what should i expect cost wise and service wise?

I've read that they will test the head to see if it's cracked...if it's good they will check for flatness and resurface if needed...and do a valve job if needed and approved.

I've got everything torn apart to the point that all that's left is removing the timing belt. Is that special tool sold by ipd to hold the engine the only option when removing that crankshaft bolt?

 

Don't do too much! This is a sickness I see here all the time. Every time you do something that's not needed you run the risk of screwing something up. Go step by step; you may not even need a machine shop! Take the head off, look for a crack in the head gasket, see what you got, then proceed accordingly.

RE the balancer removal, you could use the "rope method": get a good sturdy size piece of rope, make sure it's not fraying or coming apart, feed it into the number 1 cylinder spark plug hole, then try to undo the bolt with a half inch breaker bar. As the the engine turns, the number one piston moves up and the rope cushions it to where it can move no more giving you the resistance needed to break the bolt loose.

 

I agree with lev on sending it to the machine shop. You can measure the head for warpage just by using a square and a feeler gauge. If it isn't warped beyond spec, then clean up the mating surfaces and go with a new gasket. If the water pump hasn't been replaced for a long time, it's a good time to do that too. Do your own inspection for cracks, burned valves, etc.

If the car hasn't been overheated then it probably is all ok.

 

Time to do a headgasket! Level 8 job.


You can always do a coolant pressure test and/or cylinder leak down test. That requires Tools. Buy some or make your own. I have seen plenty of homemade solutions. You could use a spare coolant cap and put an air fitting into it. Probably even find a way to hollow out a spark plug and press/glue/screw an air fitting into it. I think I just came up with something to have a few beers and mess around with!

 

The head is off. My phone ain't cooperating with uploading pics.

There is a "clean" ring around cylinder 1 and somewhat around cylinder 2 on top of the head gasket. The top of piston 1 is black with some crusty stuff on it that I'm gonna vacuum off.

I ain't really checked out or cleaned the head yet.

Have a safe and Happy Independence day.