2001 Jaguar S-Type Rough idle/ Diesel like sound

SEAGATES5

MEMBER
2001 JAGUAR S-TYPE
4.0L
V8
AUTOMATIC
180,000 MILES

My car idles really rough and sounds really bad and there are no codes thrown. The timing had jumped and was realigned properly with the timing tools. The cold compression was about 90 psi and about 135 psi when warmed up for all cylinders. A vacuum test was done and it was a shakey 16 mm hg. Fuel pressure was also tested at 40psi. I have no idea what the problem is can anyone recommend anything?

4 Replies

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

These engine can be very temperamental. Is the oil clean and full and the proper viscosity being used? As far as the compression, I have dealt with ones that the oil has been too thin causing a low compression issue. I can't seem to find the manufacturer's specs for compression, but your cold reading seems low. As the engine warms, metal expands. Thus, that is why you get the higher reading. Plus, the vacuum seems a slight bit low. Although the book says 16 to 22 in-hg, you are on the low side. Have you checked for possible vacuum leaks?

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Sunday, May 3rd, 2015 AT 8:01 PM

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

After thought. You mentioned that the timing jumped. Were the chains replaced?

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Sunday, May 3rd, 2015 AT 8:02 PM

SEAGATES5

MEMBER
2 POSTS

Yes the chains were replaced and timing was realigned use the timing tool kit. I checked for vacuum leaks and wasn't able to find any leaks. I'm not sure what viscosity is currently in the engine but I do know that it does need to be changed. I was told that the compression is supposed to be at 180 psi min but I'm not sure how accurate that is. I did also a wet/dry compression test which came out to 90 psi dry and 130 psi after adding a few drops of oil. Does the big increase point to worn piston rings?

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Thursday, May 7th, 2015 AT 1:26 PM

JACOBANDNICKOLAS

MECHANIC
109,755 POSTS

It does indicate the rings are worn. However, in all honesty, I know these engines are a bit touchy when it comes to the oil that is in them. I had one that refused to start after sitting for 6 months. After changing the oil, it fired right up for me. And no, for any other vehicle that isn't common.

As far as the compression specs, that is possible. Also, please check timing with a timing light to make sure it is correct.

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Thursday, May 7th, 2015 AT 7:08 PM