I have a 2000 4.0 V8 Jag. It has 110k miles and was running very good until recently. I do all my own work on the car and am an experienced engine mechanic. A few days ago I tried to start the car but it suddenly acted like it had no compression. There was no unwanted noises, just the engine turning over very quickly like some of the spark plugs had been removed.

I assumed a slipped or broken timing chain so I pulled the intake manifold and valve covers off. The cams are in time and the timing chains are OK and tight. The variable valve timing setup looks mechanically OK.

The valves appear to be are clean because of detergent gasoline in USA and I also run injection cleaner through it regularly. I have looked at the intake valve sealing surface and it looks very good. I cannot see the seat. I did a compression test on cylinders 1,2,3,8 as a sample and all of the cylinders read 50 lbs of compression or less....very strange.

I have checked the valves on a cylinder of each bank by first' making sure they are closed by setting cylinder on TDC then checking cam to cup clearance at .007 inches. Then I poured about an ounce of rubbing alcohol into the intakes ports. It does not run out but seems to just sit there. So far, so good. Then I put about 100 lbs of air pressure into the cylinder. It should turn the motor over, but instead bubbles come up through the alcohol indicating leakage of the intake valves. The exhaust valves my be leaking also but for now I haven't checked. Easy to put an ear to the exhaust pipe with the cylinder pressurized, just have't done it yet.

Upon inspecting the leakage I can clearly see it is coming from all the way around both valves on the cylinder I am checking. This indicates to me that the valves are not bent. They seem to be open maybe a few thousandths of an inch. The springs are good and the buckets are free to rotate at TDC so they are not stuck.

I have rigged up a wire brush on the end of a wire and cleaned the valve and seat the best I could with no improvement. Right now I am debating whether to pull the head or not. I am concerned that if I do a complete valve job, get it running again the same problem will reappear.

I am hoping for someone out there who has had a similar experience and can help me understand why this is happening. I have tried oil in the cylinder, but it did not help.

Paul

 

Have you performed an actual compression test and seperate leakdown test yet?

Let's start there.

You can have a very,very slight problem from an over rev causing this and you won't be able see a slight bend but I don't think you can over rev this engine unless you have a manual trans and missed a gear. I've done that one at the track.

A decent compression testor and leakdown tester will set you back $100-120. Stay away from the unk at Harbor Freight etc. Even some of the stuff sold via those auto parts store circulars is junk.

Bob S.