Repairing valve seat at home....

Hey all. Ok. Ill try to keep this potentially long story short. 94 XJ 4.0 auto. Had a slightly rough idle for the past few months. Chalked it up to crappy motor mounts letting the motor move too much and just from having 120,000 miles on a recon motor. Did a compression test on it. All cylinders except #3 were great. #3 had almost zero compression. 95% leak down. When putting compressed air in, you could hear it coming from the exhaust. Pretty much convinced myself it was a burnt valve. Just got done pulling the head last night and the #3 exhaust valve was indeed burnt. REALLY bad. Like a V shaped piece of it was gone and it was distorted alittle. I had driven it two hours to paragon, through paragon, and back home.... twice in that condition. Gotta love those 4.0s.

Ok here is my question. The valve seat is obviously not the greatest. The new valve seems to seat pretty decent. I lapped it some just to see what it would look like. Does anyone know of anything I can do to clean the seat up so as to promote the valve to seat properly? I am pretty procient with a dremel... any attatchments to use on that? Or can I simply just keep using lapping compound until it works out? I can attact pictures if anyone wants them... Im trying to stay away from getting machine work done. Heck if I just plopped the new valve in it would have to run better than it was before. Any help would be appreciated. Thanks

Justin

Get a valve job. Valves are not like rings, that seat in after you replace them.

With a valve any flaw in the seat only gets worse and fast.

You cannot do it with a Dremel. That's guaranteed to make it worse, not better.

How does it look after doing some lapping in? Do you see a smooth, fairly uniform ring all the way around on the valve, and on the seat? If so you're probably okay. If there are any gaps or thin spots, that's where you might have problems.

Whether it's worth getting a valve job done depends a lot on the age and condition of the engine, and your plans for the vehicle. I've never had good luck doing valve jobs on high-mile engines. The valve job works, but soon after the rings start to show blow-by.

Sea story: Many years ago I owned a sailboat, with an Atomic 4 engine. When I put it the water in the spring, the yard forgot to replace the fuel filter, and I wasn't getting enough fuel. Ended up burning a valve, badly, due to lean burn. Engine had no power and I actually had to end the season several weeks early.

We decided over the winter to sell the boat, and the new buyer came all the way from Maryland to see it. I told him up front that I was sure the engine needed a valve job. My yard wanted too much, so he decided to take it to the new yard and have them do it, but he asked me if I could do anything just to be sure it would get him from Connecticut (Long Island Sound) through Hell Gate in New York and down to the Chesapeake Bay.

So I found an Atomic 4 valve at a crusty old auto parts store in town. The kid in the shop was a former Coast Guardsman, who knew the Atomic 4 engine. So for a case of beer he went with me to the yard, we yanked the head, found one burned exhaust valve, just as expected. Scraped off as much crud as we could with hand tools, put in the new valve, lapped it in as well as we could, and buttoned it up.

I spoke with the new owner a couple or three years later and asked him how much the valve job had cost when he had his yard do it. He said the engine ran so well on my makeshift repair that he hadn't touched it.

As long as you don't have visual holidays in the lapped area, you're probably good to go.

We used to do all our own valve jobs. You can get the grinding wheel that mounts on a drill and has the centering arbor. Other than guiding it better, that is all a shop does. I think you can get the set at JC Whitney for $20-$40 if I remember right.

old_man said:

We used to do all our own valve jobs. You can get the grinding wheel that mounts on a drill and has the centering arbor. Other than guiding it better, that is all a shop does. I think you can get the set at JC Whitney for $20-$40 if I remember right.

Click to expand...

Thats more what I was thinking. I mean... the motor runs awsome otherwise. All the other valves and seats look good besides a little bit of carbon. I want to do some more lapping and see where I stand. But I have a feeling it wont do enough. Im trying to see if anyone local has the grinding wheels. I want to get this done soon... but we'll see what I can come up with. I sort of have stroker aspirations... so maybe this repair wont have to last too long anyway. Thanks for the help... keep the ideas coming

Justin

Can you post a GOOD pic of the lapped valve seat and the new valve?

Eagle... Definetly can. I only lapped it for a minute just to see how it would work out. I want to do it some more first, and then I will post a good picture of the lap markings on both the seat and the new valve. The valve so far has a nice uniform lap mark all the way around. The seat does have a few areas that arent showing yet... but like I said I only did it real quick. I got ahold of a nice tool for re-surfacing the seat and the valve to match. So... Ill post pictures of the lapping and see what you think. Thanks for the help.

Justin

I color the valve and seat with a waterproof marker or metal dye, then lap it a bit and look, then repeat. Then decide if I´m gonna need a seat tool. The seat tool I have, is a bit sloppy in the guide and far from exact.

UPDATE: Well... I worked on the valve seat last night. I got ahold of a really nice tool for working on the seat and it really helped a lot. Unfortunetly, not enough. Even with quite a bit of metal removed, I could still see light around the valve when installed. So... after talking to a machine shop and stewing over this for a day, I decided to get a new head. Im getting one from work. $299.99 fully loaded with new valves, seals, springs etc. with a 1yr warrenty. My reasoning for this was that I also noticed that I had some warpage in the head, and some of the other valve seats were looking kind of rough.

Thanks for all the help! I think if the mesed up valve hadnt smashed up the seat so bad I would have been ok. I might still post pics just for the heck of it.

Justin

Just make sure the replacement head, if rebuilt, has new valve guides, not knurled valve guides.

Ill look into that old man.. thanks. The head is coming from Moore's Cylinder heads. They have a good rep, so Im sure it will be a good head. But I will check out the guides and see whats up. Should be in tomorrow or the next day. I might still get this thing back together before I leave without getting into a hurry...

Justin

Oh by the way... here is that valve that was messed up... Ugly huh? Its amazing that I passed emissions with no problems and made back and forth trips to paragon as well as countless highway miles. All while pushing 33's with stock gears... Gotta love the 4.0!!

wow, that thing got jacked up pretty good, i need to take picture of a valve sticking through the piston, that thing was pretty screwed up(came from a quad motor). your right, do gotta love that 4.0