burnt exhaust valve on f-350

[dlb]

on our recent 4000 km road trip, i learned that our '91 f-350 with a 7.5L gas engine burns a lot of oil. i have never noticed it to visibly burn oil but it has zero leaks. just for good measure, i performed 2 seafoam oil treatments when we first got the truck two years ago. on our trip, it probably burnt about 1 liter of oil per 250 km of highway driving.

the truck performed great on the trip but such high oil consumption alarmed me so i just did a compression test. most spark plugs looked very good but plug #2 had a little oil on it and #6 was in really bad shape, very oily and covered in hardened carbon deposits. the test results were

1 - 180
2 - 170
3 - 183
4 - 185
5 - 189
6 - 135
7 - 170
8 - 185

as you can seen, #2 is a tiny bit low but i don't think it's enough to be concerned with but #6 is substantially low and seems to point to burnt exhaust valves on that cylinder.

at this point, i'm not sure what course of action to take. the truck runs well, has ample power, and gets probably as good of mileage as a 7.5L is going to get (i managed to squeeze 10 mpg out of it at highway speeds with the camper in the box--up from 8 mpg before installing the vacuum gauge), but consumes a lot of oil. i figure option #1 is that i could just stay on top of the oil and keep driving it as it is since oil consumption the only issue. option #2 would be to have the valves redone but on a V8, i think that would be cost prohibitive. option #3 would be to have the engine replaced with a good used one from a wrecker and hope for the best. i think this would cost less than having the current engine repaired but i think used engines are a gamble. something i would consider if i went this route is to swap out the 7.5L and put in a 5.8L which came in other f-350's of the same year. i think it would still have enough power for our uses but would hopefully use a bit less fuel.

i am leaning towards option #1 until something goes really wrong, at which point i think i would go for option #3, but i want to get other forum members input. maybe there's an angle i'm not considering.

i will note that i would not be doing any engine work on this truck. it's a horrible, artless thing that i have no love for so i don't have the interest to do such work on it myself. plus it's a royal PITA to work in the engine bay with it so high up.

[irowiki]

Funny, this is what is happening to Pokey. Massive oil loss (quart per 100 miles or so), power is fine, fuel economy is mostly okay. Compression seemed fine when we tested it. 140's or 150's? Zero blue smoke.

How does a bad exhaust valve eat oil?

On Pokey we were doing option 1 but it got scary. Once she ate 2 quarts in 100 miles for no reason. Now that I sold her to dad, we're doing a total rebuild. Though if it was JUST the valves...

The 5.8 may use a different transmission?

[dlb]

i guess a burnt exhaust valve wouldn't necessarily cause excessive oil consumption unless the valve seals were also worn out, which is quite possible. so it's could also be the rings on #6 are damaged. i didn't initially think that was it since they tend to all wear down evenly but they can also fail individually.

yeah, i've been thinking lots about your saga with pokey. i'm really curious to see what you and your dad find during the rebuild.

good point about the engine/trans combo for the 7.5L and 5.8L. i haven't done any research on that swap yet so i have no idea yet if it's even viable.

[irowiki]

Oh I forgot to add, next to zero blue smoke. We replaced the stem seals.

[marlinh]

Did you do a wet compression test on that cylinder? A burnt exhaust valve wouldn't use oil as Paul mentions. You would have a dead cylinder.

Conversions are always a PITA. If you are not attached to the truck don't take it on. You'll probably want to push it into the back 40 and torch the thing out of frustration...

[dlb]

that's funny marlene, i just remembered doing a wet compression test while lying in bed, brooding about this this morning. i'll do that today (although the cylinder already appears soaked with oil as is) and post the results.

[dlb]

just re-tested #6. i left all the other plugs in this time because they're such a PITA to access. unsurprisingly, the dry test was a little lower than last night when i had all the plugs out at the same time. i just got 130 psi dry, and 140 psi wet.

i've thought about this a few times though and am not sure how useful drip down compression test results are though. it seems like pouring any amount of oil in the cylinder would raise the compression not so much by means of a better seal but by displacement.

[Petros]

if the compression comes up a lot with a wet compression test, it means worn rings. If it comes up only a little (Like yours did) it usually means bad valves. bad valves may or many not be related to excessive oil consumption, if the guide or valve stem is badly worn, it will wipe out the seal and allow a lot of oil down the guild. In most cars these wear out evenly, but strange thing with the quality control on detriot vehicles, is you can break one item out of many for no reason, even if not badly worn, while all of the other identical parts are perfectly good for many thousands of miles more. We just blew a relatively low miles Buick v6 engine. I hope never to own one of those again.

[keith]

You could have two problems, a burned valve and stuck oil control rings. Stuck oil control rings do not always cause a loss of compression, but they will pump oil into the combustion chamber to be burned. That in turn will cause deposits on the valves that can lower compression.

Soaking the cylinder in a stoddard oil like seafoam or breakfree can help, but usually do not. It takes a rebuild normally to fix this, a head job alone will not fix it and actually make it worse.

[irowiki]

I suspect stuck oil rings are the cause of the oil loss on Pokey but there's never any smoke.

[dlb]

i remembered peter talking about a snake oil called 'restore' so i did some research and found a lot of positive reviews on various forums, people giving before and after compression #s and oil consumption info. i figured at $18 it's a small gamble so i just threw some in after doing an oil change. i will keep track of the oil consumption, do another compression test after 1000 km or so, and post the results here.

i actually added seafoam to the engine oil twice. the first time was just because i didn't know the vehicle history and the second time was after i first found it seemed to be consuming oil. that was about a year ago. i've had excellent results with using seafoam to clean carbon'd up oil control rings so i think i have ruled that possibility out for now. we'll see how this goes though.

thanks all for the input so far.

[irowiki]

I tried every product I saw, the net result was I think nothing I added was able to stay in the engine long enough to make any difference.

Here's what I tried based on suggestions and hearsay etc:

Restore
a little bit of seafoam with the oil (few ounces per quart of oil as per instructions)
a lot of seafoam with the oil (the entire bottle)
seafoam down the sparkplug holes then let set for a while
marvel mystery oil in the engine oil
About ten different brands and kinds of engine flushes and cleaners
Diesel Oil (this was promising, helped at first and then it made no difference)
Running mostly ATF instead of motor oil
synthetic oil because of cleaning ability

I'm sure there were other things too. I was going to post and suggest you try a few things that could work short of a rebuild.

[Petros]

sometimes nothing short of taking it apart and replacing parts will fix it, but sometimes the adatives can help and are wroth a try if you do not want to rebuild it.

I picked up a high miles Tercel4wd in New Mexico that burned a quart of oil every take of gas (about 300 miles), and so badly fouled the #3 spark plug I had to change it out at each fill up (it would be running on three cylinders when it was time for a fill up). I suspected the valve stem seals were shot, and it had warn rings, compression was even but low (like 120-130 PSI range as I recall). The max speed it could reach with my foot on the floor was about 64 mph, even on a flat and straight hwy. I tried a couple of things on the long drive home, but none help, some made it worse (never use those spark plug extensions to prevent fouling, they cause the spark plugs to over heat and melt!). I put in a can of Restore in El Paso, and than drove on to Austin to visit my daughter, than drove it back home to the Seattle area, via Los Angeles, during July heat. in the last 4000 miles of driving after the Restore it only burned one quart of oil total, and I did not have any more spark plug fouling, and the top speed on the hwy was 84 mph, both power and economy was improved. I would have never guessed it would have worked so well, but it did, much to my surprise.

I suspect however that part of it was I put it in during the high speed driving in summer desert heat, the particles tend to settle out if it sits (you have to shake the can really well before you pour it in), so I suspect it would not work that well unless you put in right before a long drive.

[dlb]

just a little update to this. the snake oil seems to have helped! the engine has not visibly consumed any oil in the last 300 km (on the road trip it would have burned about a liter in that time), the #6 spark plug is no longer oily and covered in carbon deposits, and i did a half-assed compression test (i left all the other plugs in and only tested the #6 cylinder, but engine was warm, battery fully charged, throttle wide open) and #6 has come up from 135 psi to 165. i will continue to monitor things but at this point, i'm pleased with restore.

[irowiki]

which snake oil