Buying an e34 M5

[x_atlas0]

This is what I got back from the shop:

Good morning Walter

When we did the leak down and compression test we let the car sit over night in the shop and performed the leak down/compression test cold the next morning. It was not necessary to do a wet test because Dan could hear air coming out of the valve cover with the oil fill cap off. This indicates that it was going by the rings. If the air had been coming out of the intake or exhaust that would have indicated that it was a valve issue. As far as cost to rebuild engine, we would not have an answer until engine is taken apart. If we were to guess between 8-12,000 in labor and parts.

Is air going by the rings less serious than it going by the valves?

X_atlas0, you mentioned that a head rebuild is more normal than a bottom end for these motors. Do you have an idea of what a head rebuild might cost on an S38B36?

At this point, I am trying to avoid buying a car that will need $5000 (or more) in repairs/preventative maintenance on top of the purchase price. It seems like this car may not be the one.

-Walt

Walt-

When doing a compression test, I have seen it done cold and hot. A warm engine seals much better than a cold engine, but a cold compression test can give you better information in terms of engine health.

As far as a head rebuild goes, assuming you aren't doing the work yourself and the hard parts (cams, valves) are still good, it can be as cheap as 1500$ for new valve seats, gaskets, and decking the head+block. If you replace the valves, the price more than doubles.

sfdon-
Is that one of the 3.9L Dinan strokers? Someone in GA I met once (at Bobby Thrash's yard) had a Dinan turbo E24 M6, I wonder which would be quicker?

 

[Mike Goble]

"Leakdown percentage of what"

Shade tree opinion on what it means: you supply a given amount of air pressure ( your choice but 100psi is a typical top number on typical gauges). The tester as a second gauge that shows the static pressure on the cylinder. It's the supplies P minus what is leaking out. For example, I did my jag engine this weekend. Inlet gauge at 100psi, and cylinder gauge at 98psi. Therefore my leakdown was 2% on that cylinder. Obviously, using 100psi as he reference makes expressing the result as a percentage eAsier, and its correct to present it as a relative amount anyway.

Why does your engine only leak 2% as compared to 10%? Who determines what 2% means?

 

[sfdon]

Old school quicker goes to the high compression. Turbos were best on the open road.

 

[WALTER]

I decided to pass on the M5. Ultimately, the mileage was a little high for not having a rebuild yet, so it was probably going to be due sooner than later given the numbers on the compression and leakdown tests. Aside from the engine, the inspection turned up about $2500 worth of other work to do, so I could almost count on nearing the asking price in repairs/maintenance as soon as I purchased it.

thanks for all the advice.

Walt

 

[JFENG]

Goble

Why does your engine only leak 2% as compared to 10%? Who determines what 2% means?

Why?
Ummm, maybel because my rings and valves were properly fitted are in good condition....

Who determines?
I think the definition of what constitutes "ok" is fuzzy.
I did my test "hot" and followed with a wet test to see if any numbers changed. IMHO, a motor in good shape and without a lot of crap on the valve sought to seal up pretty well. Is losing 10% of the available power stroke compression ok? In my book, it isn't ok because I just paid $xxx for higher compression pistons and I want to see higher compression at tdc.

 

[Mike Goble]

I can make a leakdown tester that will have the best engines leaking 50%. I can make a leakdown tester that will have the worst engine leaking 10%. My question is who determines what 10% leakdown is?

If you do a compression test, what units are you using? PSI - and we all know what PSI is based on established standards. When you calibrate your speedometer, how do you know it is correct? A mile is a mile, an hour is an hour.

% isn't a unit of measure unless you know what you are measuring a % of.

I have a leakdown tester that measures actual air flow through the cylinder using a rotameter.

 

[JFENG]

I agree but ...

Why does your engine only leak 2% as compared to 10%? Who determines what 2% means?

I can make a leakdown tester that will have the best engines leaking 50%. I can make a leakdown tester that will have the worst engine leaking 10%. My question is who determines what 10% leakdown is?

If you do a compression test, what units are you using? PSI - and we all know what PSI is based on established standards. When you calibrate your speedometer, how do you know it is correct? A mile is a mile, an hour is an hour.

% isn't a unit of measure unless you know what you are measuring a % of.

I have a leakdown tester that measures actual air flow through the cylinder using a rotameter.

Compression is expressed in psi afaik.
Leakdown: as I said, I use 100psi. Sure I could put 5000atms on it and it might leak much worse, or 15psi and get a good reading on anything. In fact, for a 10:1engine with less than a full race cam, compression ought to be up in the 170-190psi range so I think 100psi is a bit low and I'd like to use 185psi ( but my gauge doesn't go that high). All five of my 9:1 to 10:1 engines have fallen into the aforementioned range with hot street cams. None of them showed more than five percent leakdown re 100psi wven after 10k miles. So based on my limited experience 2% is good and >10% is bad. Also, I have a tired 8.8:1 motor with a cracked compression ring. It measured 20% leak down in this condition and compression was also nearly 20% low relative to the other three cylinders (165psi, hot and wet).

Anyway, this is very limited data so I agree that idiots like me can't be trusted to do them or understand the results. heck, my gauge has NEVER been calibrated against a NIST certified reference! The real pros like you know how to do them and understand how to relate the numbers to engine condition. That's why I pay professionals to do my PPI's

 

[x_atlas0]

I decided to pass on the M5. Ultimately, the mileage was a little high for not having a rebuild yet, so it was probably going to be due sooner than later given the numbers on the compression and leakdown tests. Aside from the engine, the inspection turned up about $2500 worth of other work to do, so I could almost count on nearing the asking price in repairs/maintenance as soon as I purchased it.

thanks for all the advice.

Walt

That doesn't surprise me. Like some other cars we know, I have heard the E34 M5 called a "15k car"; pay it now or pay it later.