My 86 Ram ran horrible when I bought it back in early July. It was completely stock and wasn't well maintained at all. It had a lot of various vacuum leaks, a leaky carb that was smeared with silicone, and hadn't been tuned up in a long time just to name a few things. I checked compression and it had about 35 psi on cyl #6, the other 5 cylinders were all 120-135 psi. I did a leak down test and all the air was coming out of the exhaust....I pulled the head and found a burnt exhaust valve on #6, bad enough that it had a v cut in the face.

I took the head to a local machine shop and had a complete valve job done, guides knurled, and new valve seals put in. While that was going on, I rebuilt the carb, fixed all the vacuum leaks, and tuned it up. The truck ran great for about 2 months after that. Around the end of September, it started running a little rougher than normal and the MPG dropped by 1.5 - 2 MPG. I didn't check compression again at this point, but did check everything else that I could think of.... tune up, timing, carb adjustments, vacuum leaks, etc. Everything I checked was spot on where it should be... the plugs looked ok, they were slightly lean (the plugs had a "dirty white" color) but nothing that would seem to indicate a problem.

At this point I began to think it was carb related, plus I wanted a super six setup anyway. I bought a super six setup, rebuilt the BBD, and put it on. The truck seemed to run better, but still wasn't "right". I checked everything again, carb adjustments, timing, vacuum leaks, new plugs, new plug wires, new coil, etc....nothing fixed the rough running and MPG. The plugs, after the super six swap, were more tan (the way they should be) than before the swap and looked fine. Finally, since I was out of other things to look at, I checked compression again about a week ago. Cylinders 1-5 were 120-135 again and #6 was 70 psi.

I did another leak down test on #6 and found air psi coming out of the tail pipe again. I pulled the head again and the exhaust valve looked normal while it was still installed. I took the valve out and the face was pitted all the way around the face, the seat looked great. I took the head back to the machine shop and they can't figure out why this happened. The valve job literally only had about 3,000 miles or so on it when it started running bad again at the end of September, which I assume now was because of low compression on #6.

The machine shop guy is of the opinion that their valve job was done correctly and the most likely cause of this valve burning again is from the engine running too lean. I feel like it would have to be extremely lean to burn a valve again so quick, and it seems odd that it's the same cylinder as it was 3,000 miles ago. We aren't "fighting" about it or trying to point fingers at each other.... but neither of us want it to happen again by the next oil change either lol.


What are your thoughts?

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1970 D100 "Dude"
SL6 / 3 speed manual

I agree that the engine is running lean, but #6 shouldn't be any leaner than #1. Do you have power brakes? The vacuum source for that is the #6 intake runner and if there is a leak there #6 will run lean. Hopefully the PCV valve isn't connected there. The PCV valve should be connected to the carburetor.

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Joshua

The PCV is plumbed to the carb.

It does have power brakes and the brake booster hose is plumbed into the vacuum fitting on the #6 runner. I don't think the booster is bad or leaking vacuum internally. You can pull the hose/check valve off the booster with the engine shut off and it definitely holds vacuum in the booster..... in other words when you pull it off the booster it has a loud sucking sound, which tells me the booster ok vacuum leak wise. The hose from the intake to the booster is new as well.

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1970 D100 "Dude"
SL6 / 3 speed manual

Hydraulic cam? I have to wonder if the valve geometry is off. Any thing out of order in the lifter or rocker that might not allow the valve to close with the lifter's internal piston about halfway in its travel? Were the valve stem lengths above the head set to the correct height? One stem too long would throw off the geometry and could allow the valve to barely close and be prone to burn.

Also, were the original valves re-used, especially #6 exhaust? If re-used and ground down a lot, then there may not be enough valve margin (valve head meat) above the valve face. This valve margin needs to be there to manage the heat at the valve face.

It is a hydraulic cam.... everything looks normal with the rocker shaft and pushrod, the lifter looks normal from what I can see looking down at it with the head off but I haven't pulled it out for a closer look yet. I checked the valve stems with a straight edge when I pulled the head this time, they were all pretty close, maybe .030 or so difference in some but nothing extreme.

When the machine shop did the valve job back in July, they said that I needed 2 new exhaust valves.... the other 10 valves were reground and reused. I don't know for sure that one of those 2 brand new exhaust valves was installed in the #6 cylinder, but I assume that it was. I don't recall looking at that specifically before I put the redone head back on the engine in July.

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1970 D100 "Dude"
SL6 / 3 speed manual

I find this very interesting. I have an interest in the hydraulic motors. I have taken apart several hydraulic motors and talked to others who have torn them down and I have seen what I believe to be an unusually high amount of burned or damaged exhaust valves in the hydraulic motors. Of the last 5 I have worked on, three have had burned or even badly cracked valves.

This is odd because for 15 years I had worked on slant sixes and never saw a cracked or burned valve. But when I started playng with the hydraulic motors I began to see valve damage problems in about half the engines I took apart.

Did the valve metallurgy change when the switch was made to hydraulic lifters? Did the air injection directly onto the back of the exhaust valves raise the temperature on the back of the valves or somehow otherwise damage the valves? Were the tolerances for valve stem heights not tight enough from the factory?

Interesting questions. Might have me thinking twice about building a hydraulic motor.

I assume this has all the original 'lean burn' stuff on it? If it is running a little bit leaner than it was designed for there could be problems.

I'd look carefully at all the other exhaust valves. They could all be showing signs of damage but they would not necessarily all fail at once. In other word, one of them will have to be first even if the problem is not confined to the #6 cylinder.

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Now that you mention it, I wonder if the lean burn system is the issue. Air injection came into use in the 70s back when the slant still had mechanical lifters. I bet the issue is years of a malfuntioning lean burn system running the engine too lean which leads to hastened burning of the valves.

But that doesn't explain why the valves repaired during the head rebuild would fail so quickly.

Chief195- were you running a lean burn system after the head rebuild but before the Super Six swap?

Reed,

Those are my thoughts exactly.... Many things could have caused the first valve to burn.....my concern or confusion is why it burnt again 3,000 miles after a fresh valve job. Especially considering I also fixed the vacuum leaks, rebuilt the carb, and tuned it up at the time the head was being worked on the first time....So it should have been tuned in better than it had been in years.

The truck did have the lean burn system still installed, after the initial head rebuild, for probably 3,500 miles or so. It was the lean burn system that only controlled the ignition, in other words it did not have the feedback carb.

Once it started running bad again in September, I upgraded to the HEI system and eliminated the lean burn system all together figuring that may have been the culprit for the rough running and suddenly bad MPG.

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1970 D100 "Dude"
SL6 / 3 speed manual

Does your engine have the welded aluminum intake manifold?

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Joshua

The 1 bbl manifold was cast aluminum, I don't see any welds on it anywhere. I'm not sure if there was another type of aluminum manifold that was welded or if I misunderstood what you're asking.

The super six manifold that is on it now is cast iron.

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1970 D100 "Dude"
SL6 / 3 speed manual

80s truck manifolds are *usually* the cast aluminum versions. The 70s-early 80s car and truck aluminum intakes were two piece nits electronically beam welded together. The two piece e-beam units are known to have porous welds and leak.

Did you check the old intake for leaks? I have seen several aluminum intakes crack right by the EGR port on the floor of the intake.

Is the power brake vacuum fitting in the #6 intake securely sealed?

Did you have the head shaved? Just another factor in the valve train geometry to consider; shaving on the hydrualic valvetrain can take that much out of the lifter piston travel. IMO, the .030" variation in valve height is fair, not great.

Have you measured the actual installed valve height to spec? That needs to be done to be thorough; geometry may well not be the issue at all but to be complete, you need to check it all out consider all factors. Having said all that, if the lifter was close to bottoming out, then it would likely have run erratically rough from the start.

It wasn't shaved as part of the valve job I had done, the machine shop guy checked it for warpage and said it was ok. That's not to say that it hadn't ever been shaved previously, but I doubt that it had.
I'm hoping to get the head back from the machine shop today. I will take a closer look at the installed heights and make sure they are good.

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1970 D100 "Dude"
SL6 / 3 speed manual

Could there be a restriction in the cooling system by #6 exhaust valve?

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Dave

1977 d-200 crew cab ex-army pickup wants it's /6 back
1962 Valiant 2 door, 170, three on the tree
1972 d-100 parts truck
80 volare wagon now a parts car