Dougs951S

Just like the title says, for a long time now (probably a year) I've had a burning oil smell coming from the car while under high vacuum decel in gear. oil is getting sucked out somewhere and burned (I assume) under these conditions. I've noticed it getting worse in the past month or so, so I want to fix it. What am I looking at here. The head (please no), AOS seals (easy) Something different?

hacker-pschorr

Valve guides / seals.

How many miles on the engine?

Droops83

Unfortunately, it sounds like a classic symptom of worn valve guides and/or seals. This one is difficult to prove without taking things apart as it won't show up in a compression or leakdown test unless the valve guides are very badly worn and the valves aren't seating properly.

How many miles on the head/engine? What is your oil consumption like (how many miles per quart added)?

I finally had time to rebuild the head on my '86 w/ almost 160K miles a few months ago, as I had been monitoring slowly worsening oil consumption during normal street driving over the past couple years. It was using over a quart per 1K miles during normal street driving and could see oil pooled in the #1 cylinder through the spark plug hole after the engine sat overnight. Compression was good at 150 across the board and leakdown was likewise good at ~5% on all cylinders. I track the car a lot and it actually wouldn't use much oil on a full track weekend (maybe 3/4 quart) which is well within acceptable limits on a 951 with such high mileage.

In the last few months I noticed a bit of stumbling on cold startup and leakdown on #1 had crept up to 7%, so I pulled the trigger and installed the spare head I'd built up. Indeed the exhaust valve guides in the original head were shot and the #1 exhaust valve face and seat were starting to pit, and the valve stem seals were rock hard. Still, I can't complain as the original head/gasket went 28 years and 160K miles (10K+ track) without a problem.

A breather/AOS problem would likely show up more under load/boost as that is when blow-by happens.

A compression/leakdown test might be a good start. Pull the spark plugs and look inside the cylinders after the engine has sat overnight and see if there is oil pooled on the exhaust side cylinder wall area and report back.

Dougs951S

Oil pooled on #4 sparkplug. looks like I'll need to rebuild the head soonish. Funny thing is the car doesn't smoke, even on cold start up after sitting overnight. (Well gee, I guess not that I've noticed.)

txhokie4life

What he said....

Take the heads to Brooks Elliott. Not the cheapest (but not that bad either),
knows his stuff and has always done a great job AND is honest.

Found some issues on both West's and Jeff's 951 heads.

Mike

Dougs951S

Well, how do I know if its the guides or just the seals? I have a whole set of new oem valve stem seals and I can put those on without pulling the head.

lee101315

The valve guides have to be replaced, especially with higher mileage. Replacing just the seals for now will reduce oil consumption for only a few thousand miles if the valve guides are worn out.
In addition to the oil consumption, worn valve guides also cause the valve to not seat properly, which also leads to a loss of compression and seat overheating ( the valve can't transfer heat to the seat ) which could lead to a burnt valve.

Remove the head, do a minor port and polish with a Dremel kit, and send the head to a machine shop for fresh guides and a valve job. Your engine will run much better....

Dougs951S

What's the average price for getting a head rebuilt with new guides, seats, new stem seals, and a valve job + decking if need be. Am I in the 1k+ range, the 500+ range, ect? School just started this week, I had to buy all my books, I have basically no money at the moment. It might be more economical for me to put a good N/A head on the car in the mean time until I can afford to rebuild my turbo head the way I want with big valves and a nice port job.

Crazy Eddie

Hey Guys
I have had issues with oil consumption as well with my engine. After reading this thread I am more and more leaning towards the idea that my issue might be related to the valve guides/seals etc....
I am wondering what was your vacuum reading were when you had this issue ?
Would this affect your vacuum readings ?
Mine seem to peak at around 16 when the car is fully warmed up....
I don't believe I have a vacuum leak ?
Thanks in advance
Regards
Ed

Dougs951S

Eddie, i'm glad you brought up the issue of vacuum because by all accounts, my car pulls REALLY healthy vacuum for a car that apparently is in need of a head rebuild. I see 14" on an ice cold start, 18" warmed up, and 22-23" on full decel. No vacuum leaks to speak of. Idle is 900 rpm and i'm 600 ft above sea level.

Droops83

Eddie, how many miles on the head/engine?

Just as worn valve guides don't normally affect compression/leakdown, they likely wouldn't affect vacuum readings much unless they were REALLY bad, to the point where the valves aren't seating properly and compression loss occurs. You'd probably know if if this were the case.

A compression/leakdown test is a good start. Worn piston rings will show up on a dry-then-wet compression test or a leakdown test. Excessive oil in the turbo pipes/IC are another clue, as worn rings lead to increased blow-by (though some oil in there is normal on these engines).

txhokie4life

If say $1k maybe a tad more.
You can pays little less at any old cylinder shop
But brooks is your man locally.

All the 944/951 folks in Austin I know use him.

Mike

blown 944

I'm surprised that no one has mentioned the turbo and the vacuum to the turbo mount. I've had this same issue with a few garret turbine seals.

Have you changed the AOS vent line?

Chris White

The usual causes would be rings, guides or turbo.
If the leakdown is good that rules out the rings....and you are left with the guides or turbo.
Hot side turbo leaks tend to smoke best when the pressure in the exhaust is low and the vacuum is low. on deceleration you have low ehaust pressure but high vacuum - at idle you have low exhaust pressure and medium vacuum (more likely to smoke). So if you car does not smoke it idle that would normally rule out the turbo.....leaving the guides as the culprit.

If you like to run higher revs you will wear out the guides, the heavy stock valves put a lot of wear on the guides. An aggressive track driver can put noticeable wear on the guides in one season - especially if they run over the stock red line.

If you pull the head and send it out for just the guides and seals it can be done for $250-$300....but the 'while your in there' effect can add to that!

To be honest about big valves and porting - there are two choices that make sense to me. Either just a minor clean up and a good valve job or do it the right way...witch can run about $3k. just dropping in bigger valves or waving a grinder around in the ports is not worth it, it needs to be done right to make a real difference.

As always - it depends on your goals. I have built enough 370 - 400rwhp 2.5 liter engines to know that you an get there without big valves and porting. If you want more then head work starts to become a good solution.

Or you could just say 'screw it, I didn't want any money in the bank anyway' and add a 16v head!The 16v heads don't have a guide wear issue!

Dougs951S

Thanks everyone for chiming in. As always, forgive me if I say something stupid. Mike, thanks for the reference, I've been needing a good local shop that wasn't in Houston. Sid, every last hose in the car has been replaced in the last 2 years with silicone and I have deleted the venturi. The AOS is plumbed directly into my J-boot. The car does not smoke at idle, actually it never smokes which is weird. The turbo was rebuilt recently, it has absolutely zero shaft play in any direction. My compression is strong, 145-150 psi on all cylinder when warm which I thought in conjunction with my aforementioned vacuum numbers and the fact that I get very little blow by (very little oil in my hard pipes) was a good indication my rings were in excellent shape. I have never done a leakdown test on my motor, so that will be my next thing but I think this all points to the issue being in my head. Being that my rings seem fine, terrible leakdown would be a surefire indictation of the guides being done? Chris, I'm not too aggressive with the car, I try to keep it inside factory red line more out of consideration for my stock rods than anything else. I'd love a 16 valve car but I also need to drive the thing on public roads and I feel like big compressor + stock stroke + 16 valve would = no torque where I need it. My goal for now is a 375 rwhp 2.5 liter and I'm pretty sure I'm nearly there already. If I fail to meet my goals when dyno time comes, I have a brand new shiny Holset HX35 and all the parts to install it...