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Disappearing Oil. I'm out of ideas....91 TBI 350

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Big6ft6

1/2 ton status
Joined
Apr 26, 2010
Posts
251
Reaction score
6
Location
Madison, WI
I recently purchased a 91 k2500 with a TBI (replaced my 89 burb). 163k miles. All appearances suggest it was a very loved and well maintained truck. Runs and drives wonderfully, closest thing I've owned to a new vehicle.

However I'm losing oil at a pretty good rate and I can't figure out where it is going.
-VERY clean engine, no signs of oil on exterior of engine (at least where I can see)
-No spots on the driveway.
-Spark plugs clean (although they're fairly new)
-No blue smoke that I can see.
-I just bypassed engine oil cooler last weekend as there was signs of small leak in one of the cooler lines but it didn't help.
-I did a compression test and I have 150psi +/- 2psi in each cylinder
-PCV is sucking good vacuum

I'm losing about 1 hash-mark on the dipstick every 20 miles. So it is going from "full" on the dipstick down to the "add" on the dipstick in 300-400 miles.

Any ideas?
 
I would think you would be able to see some smoke behind you when driving if losing a quart in 300 miles. If it's not leaking it has to be burning it
 
Big Pappa , That is what I would think too!

I would still see the blue smoke even with a catalytic converter right? (It seems I have on my other vehicles when they burned oil) I've tipped my passenger side mirror down so I can see the exhaust while driving and I see nothing.

Only other idea I can think of is if it is dripping on the exhaust y-pipe at the back of the engine and burning off before it leaks on the ground. I do smell a "hot engine" type smell when I park the truck, but it isn't too strong...and I can't see any evidence of this on the exhaust, but of course I can't see everywhere in there where it gets tight.

I've laid under the truck while it is running trying to notice something, that is how I saw the leaking oil cooler line. The radiator fan would blow the drip from the cooler line back on the power steering hoses where it would cling without dripping on the driveway.

Only other strange symptom with the truck seems to be excessive soot in my tailpipe. It is very black inside the tailpipe, and the exhaust is newer from cat back. I can see some soot has leak out around the joint where the new exhaust was clamped to the old. But soot usually has to do with unburnt fuel, not oil right?
 
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Ehem, you found where it's going.
No unburnt fuel will be very light film of black carbon, if it's more like soot it's burning oil and bad.
I have no idea though how with 150 PSI you are getting the oil thru to burn it, unless it's an internal leak from the intake.
How is you PCV valve working?
Big6ft6 said:
Big Pappa , That is what I would think too!

I would still see the blue smoke even with a catalytic converter right? (It seems I have on my other vehicles when they burned oil) I've tipped my passenger side mirror down so I can see the exhaust while driving and I see nothing.

Only other idea I can think of is if it is dripping on the exhaust y-pipe at the back of the engine and burning off before it leaks on the ground. I do smell a "hot engine" type smell when I park the truck, but it isn't too strong...and I can't see any evidence of this on the exhaust, but of course I can't see everywhere in there where it gets tight.

I've laid under the truck while it is running trying to notice something, that is how I saw the leaking oil cooler line. The radiator fan would blow the drip from the cooler line back on the power steering hoses where it would cling without dripping on the driveway.

Only other strange symptom with the truck seems to be excessive soot in my tailpipe. It is very black inside the tailpipe, and the exhaust is newer from cat back. I can see some soot has leak out around the joint where the new exhaust was clamped to the old. But soot usually has to do with unburnt fuel, not oil right?
Click to expand...
 
Soot means oil?? Woops. Everything I've read about plack tailpipe says it is from running rich. Yeah, I actually left a black mark on my mom's garage when I pulled away from her house the other day. But the strange thing is I can't see anything coming from the exhaust when idling or driving so it isn't like I have a cloud of black smoke like a diesel.

I was suspicious of the PCV because my throttle body is oily ( there was even wet oil on top of my injector plugs) as if oil was coming up thought the vent hose from the driver's side valve cover and spraying into my throttle body.

But when I pulled PCV valve out of the passenger valve cover and put my finger over it, it has good vacuum throughout the rpm range. So I figured maybe the oil on the throttle body was older than I thought and ruled out the PCV.

What is still buggin' me about the PCV is the fact I couldn't detect any vacuum from the driver's side valve cover:dunno:. If the PCV is providing vacuum to the crankcase shouldn't I be able to feel it if I detach the vent hose on the driver's side valve cover and put my finger over it?

And what about the spark plugs? If I was burning this much oil wouldn't my spark plugs oil up pretty quickly?
 
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Valve guide seals, mine on the '89 leaked pretty good and would start smoking a bit even during longer than normal idle. Smoked pretty good at start up all the time.

Friends truck did the same thing, we replaced the seals and it stopped.

At 160k miles, you can pretty well bet those seals are dried out and leaking.

Mine had 165k miles when I got it, got it up to 250k before the new engine went in.
 
Big6ft6 said:
Soot means oil?? Woops. Everything I've read about plack tailpipe says it is from running rich. Yeah, I actually left a black mark on my mom's garage when I pulled away from her house the other day. But the strange thing is I can't see anything coming from the exhaust when idling or driving so it isn't like I have a cloud of black smoke like a diesel.

I was suspicious of the PCV because my throttle body is oily ( there was even wet oil on top of my injector plugs) as if oil was coming up thought the vent hose from the driver's side valve cover and spraying into my throttle body.

But when I pulled PCV valve out of the passenger valve cover and put my finger over it, it has good vacuum throughout the rpm range. So I figured maybe the oil on the throttle body was older than I thought and ruled out the PCV.

What is still buggin' me about the PCV is the fact I couldn't detect any vacuum from the driver's side valve cover:dunno:. If the PCV is providing vacuum to the crankcase should I be able to feel it if I detach the vent hose on the driver's side valve cover and put my finger over it?
Click to expand...
I am sorry but I could be dumb as a rock here but I thought a PCV should not have vacuum, but actually release pressure from the valve covers.
If it's having vacuum you need to see what is causing this, but it's definitely the cause of your oil going in your engine and burning.
And you won't see a black cloud with oil burning but greyish white
 
I'll second valve stem seals. That is what my 91 does.

FWIW, my truck does blow some smoke when I drive, but I can NEVER see it in the mirrors. Have someone drive your truck and you follow behind and you'll probably see it very clearly.
 
I've thought about the valve stem seals too, but I assumed I would see blue/white smoke and my plugs would be fouled. I will follow my truck while someone else drives and watch.

Can you see the seals if you take the valve covers off or are they completely obstructed by the springs?
 
Big6ft6 said:
I've thought about the valve stem seals too, but I assumed I would see blue/white smoke and my plugs would be fouled. I will follow my truck while someone else drives and watch.

Can you see the seals if you take the valve covers off or are they completely obstructed by the springs?
Click to expand...

Seals are pretty much hidden under the springs and rockers, just about a full tear down to get to them, it's even slightly tricky to use the air compressor method, watched a friend do it on the other friends truck, I don't like it.

Pressurize the cylinder, remove rocker arm, push spring down making sure not to break the seal from pressure the air compressor is pushing, release the locks, remove the springs, dig out the old crusty seals, install new umbrella style seal, reinstall springs, push spring down, fight getting the locks back in place,,,, it's a tedious job really from what I witnessed.
 
its really easy with the right tools. not just the proper tools, but there are some really nice valve spring tools that are small and fit alot better than others. i have 3 different ones that i use depending on what engine im working on. i change alot of springs on hemi's.

oh, x10 on valve stem seals.
 
Thanks everyone, sounds like quite a consensus on the valve guide seals.

This is definitly something I would've tackled myself a year ago, but I now have two kids and one is only 7 weeks old, the wife is not a huge fan of me dissapearing for a few hours in the driveway. (actually if I predict a few hours, better doulbe that)

I'll probably call and see what it would cost to have a shop do this. They should be faster than me too. But if it is too much I might have to hold off until I can do it myself.

Is there anyway to confirm this is the problem without doing the job? How many miles would it take for the plugs to be fouled? When I pulled them to do the compression test they were sparkly clean. But the PO had said he just changed them (maybe becuase they were fouled?)
 
The engines in these years of trucks are known for the seal issue, they all do it, they even have a newer design of seal to prevent it in the future. I'm gonna bet though, with the amount of work involved in replacing the cheap cost seals, that the labor alone is going to be several hundred dollars.

Mine would burn through oil, but not nearly as much as yours did and as far as plug fouling, if the previous owner had JUST changed them, to sell the truck, well, that's WHY he changed them, so it would run decent during the sale.

Give it a few weeks and I'd be willing to bet they will foul out.
 
FWIW, I *think* many smaller shops will do this for a few hundred bucks. For some reason I think my shop said it was $300 out the door. That seems like a bargain to me ...
 
If you were burning half a quart in 1500 miles I would say maybe the seals, but burning a quart in 300 miles is excessive, you could have bad guides already and changing the seals will not last long because the guides are worn out and will ruin the seals real quick.
Just my 2 cents.
Big6ft6 said:
Thanks everyone, sounds like quite a consensus on the valve guide seals.

This is definitly something I would've tackled myself a year ago, but I now have two kids and one is only 7 weeks old, the wife is not a huge fan of me dissapearing for a few hours in the driveway. (actually if I predict a few hours, better doulbe that)

I'll probably call and see what it would cost to have a shop do this. They should be faster than me too. But if it is too much I might have to hold off until I can do it myself.

Is there anyway to confirm this is the problem without doing the job? How many miles would it take for the plugs to be fouled? When I pulled them to do the compression test they were sparkly clean. But the PO had said he just changed them (maybe becuase they were fouled?)
Click to expand...
 
I was gessing more like $500 for the seal job, assuming parts would be about 1% of that cost but what can you do. :dunno: I just had my first baby, it is 7 weeks old today, I'm so freakin' tired, and my wife and I are both about one wrong word away from killin' eachother (she is amazing, she is doing more of the work than me!). My tools have been put to use assembling baby furniture the last few weeks!

At this point I'd run to my mechanic if they'd do it for $300 and I knew this was the issue. If it got to $600...then id have to have a few beers first...then I'd still probably just pay it.

The PO only had the truck for a few months which I know is one of the biggest red flags when buyin' used, but the truck is in such great shape for Wisconsin that the price was worth it even if I had to eventually replace engine/tranny. I figured if I could get a couple years use before I had to do an engine or tranny swap I'd be quite happy. I've installed one of these engines before in my driveway, the second time should be easier right?

His story was he bought it from an old farmer who had it since 2004 and hardly put any miles on it. (141k to 161k in 8 years!), he said the old guy sold it becuase it had developed a stumble at idle. So the PO replaced "cap, plugs, wires" and it "cleaned right up". I didn't ask what the plugs looked like...but they very well may have been coated in oil. He had just placed an order for a brand new chevy when I bought this from him. He claimed this truck was just a temporary truck till he saved for the new one which seemed plausible to me (he never registered it). He was a young buck (younger than me) just married with baby, and seemed honest, not too mechanical. I've bought and sold too many old vehicles to count, and he passed my sniff test, but it is always a gamble which is half the fun.

I think I'll call and see what the seal job will cost, if the guides are that worn I will retire the truck to "truck duty only" for a few years and get a new (used) minivan for the fam as my regular driver! :D
 
Here's something that occurred to me as well. I vaguely recall, or maybe I am making this up, that someone did a valve job on their engine. Unfortunately, the rings were so bad that with the increased pressure from the new valves, a significant amount of oil was now being pushed past the rings and made the truck smoke so badly he could not drive it.

Now, I don't know how you can check if that really is a real issue, or if it is how to check to prevent it, but it seems like something plausible to me.

But then again, I tend to be an idiot most of the time ...
 
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