VR Engine Issues any help appreciated!

[Samwain 10]

Hi guys, my VR is having a few 'symptoms' so to speak and I need some guidance as to what may be the issue. The car is burning oil, particularly when you put your foot down and get above 3k rpm. Despite this, up until recently it ran fine. It's began bogging when low down and power is down. My local garage have identified issues with the crank sensor and a subsequent overfuelling issue, but having done a compression test found cylinder 2 was down at 120psi - the others all at 140 (I thought this was within tolerance??). It can be particularly tappy as well now, this seems to come and go. I guess my questions really are - is there anything more likely to be causing the oil burning and could the issues be related? I know it's probably hard to answer until the engines opened up but I'm just looking for likelihoods. Thanks.

[seanl82 23]

I thought 160 was the lower limit, but all being 140 psi is certainly low, and the 120 is very poor tbh. Mine when last tested were between 165-175 for instance, on an engine with 148k and zero internal or gasket work in its history.

[Samwain 10]

Hi Sean thanks for your reply. I read somewhere there was a lower limit of 109psi but how correct that is i don't know. I'm struggling to find any official figures for what is and isn't within limits. Thanks

[Rado87 10]

Have you got any leaks? Maybe round the back of the rocker cover out of site, burning off on the exhaust maybe? Also, my one used to burn a bit due worn valve seals. Refurb head has sorted that.

[Rado87 10]

There's a thread on here that states 11-13 bar (175 psi)for a new engine and 7.5 (109 psi)lower limit

[seanl82 23]

109 can't be right, going from what you've just said that's like 40% down.

[Rado87 10]

It does seem a lot. There's a post on hear "vr6 compression test results" stating Bentley manual figures.

[Samwain 10]

Hi Rado thanks for your help. No leaks from what i can see, certainly nothing ends up on the driveway. Sorry for the properly ametuer questions, but would a refurbed head be likely to have any positive effect on compression or is it more likely deeper lying issues?

[Rado87 10]

I'm fairly mechanically minded but feel free to correct me if I'm wrong anyone, but yes, if the valves/seats are worn your compression will suffer. It stands to reason that if the valve isn't closing properly then any oil making its way into the cylinder will burn off. You tend to notice it from cold where oil seeps through over night and is burned off in the first few minutes of running.

[Samwain 10]

Thanks again rado, this is all really helpful. Final question for tonight i think - am i right in saying the 2.8 vr6 head is the same as the 2.9 and I'll have no issue fitment wise swapping everything over?

[Rado87 10]

No worries mate. If I can be of any use, no matter how limited, I'm more than happy to help. The help and support I've had on hear has been brilliant. Anyway, I couldn't give you an honest answer on that one mate. I don't see why they should be different but you never know! What's the mileage on yours and how much oil are you needing to put it in it?

[Samwain 10]

Cheers, it's really appreciated. Engines on roughly 135k, in all honesty it's hard to say how much oil I've been putting in as it's recently had an oil change and its been driven pretty infrequently by me since, because of the issues I've been having. I just monitor the level every fortnight. I'm a couple of litres into a bottle i bought a couple of months ago but as i say it has had a full change it the middle of that period.

[fendervg 33]

At that mileage you are probably into head rebuild territory depending on how the car was looked after and serviced over the years.

[tonylam 10]

I have just pulled this from vwvortex.com

 

EVALUATION OF TEST RESULTS

According to the MKIV Bentley manual, the compression specifications for the 12v VR6 engine are as follows (the values you obtain during your test will depend on both the condition of the engine and the accuracy of the pressure gauge used):

 

- new ------------------------------------------------------ 10 to 13 bar (147-191 psi)

- wear limit ------------------------------------------------ 7.5 bar (110 psi)

- maximum difference between cylinders ------------------ 3 bar (44 psi)

 

Here's a quote from the MKIV Bentley manual (page 15d-2) which talks about compression values which are lower than spec ...

"Low compression suggests poorly sealed combustion chambers. Compression pressures which are relatively even but below specifications indicate worn piston rings and/or cylinder walls. Low but erratic values tend to indicate valve leakage. Dramatic differences, such as good values in some cylinders and very low values in one or two cylinders are the sign of a localized failure, such as a burnt valve or a failed cylinder head gasket."

 

From past experience of racing if you have a significant drop in pressure and the only major symptom is burning oil it points to

1. valve related. poor seating combined with a poor valve seal

2. cylinder related. poor seal between cylinder rings and the cylinder wall. if this has come on suddenly as you say i may be considering a failed ring.

 

it does seem like a general freshen up may be in order. it is so much easier on my lads MX Bike though.

 

hope this helps

 

tony