Repeated burned exhaust valves -94 GS

I've had this 94 DOHC GS since rolling out of the showroom in 93. Burned first exhaust valve @ about 120k. Now I get one about every 20 - 30k miles.
Now have 250k miles
Just burned another exhaust valve (4th time) random cylinders..
I have used 2 different machine shops
I have used Mitsu valves and aftermarket from one of the shops
Never overheated
Runs a little rich (a little smut at exhaust pipe)
Using prem. fuel as required

Being OBD I ....I haven't run codes yet. MIL comes on sometimes w/ city driving but resets off on the open road (OBD I inspection not required in my area)

Could the EGR (maybe stuck open) cause extra heat / these valves to fail?

Thanks for any help!!....I'm "burned-out" on this...

http://www.aa1car.com/library/ar1192.htm

Thanks for the link!.....yes, most everything I read on exhaust valve failure leads to the valve not cooling properly, or quality of the valve / seats.

I've tried two different machine shops and two different sources for parts. Now I'm thinking about the possibility of a restricted exhaust system or maybe running a little bit cooler thermostat?
anyways....thanks for the come-back...

upgrade to stainless valves if the problem persists, or it may be just worth it to do it now. check for a clogged cat as well.

link to stainless valves
http://cgi.ebay.com/ebaymotors/4G63-...item588c560ce2

sounds like a plan..... I've got the head off now .......definitely need to do something different this time...

like your 96!....I wish mine was a 5 speed. the auto tran is "crazy" ......has been since it was new.

Quote:

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Originally Posted by StockGS

sounds like a plan..... I've got the head off now .......definitely need to do something different this time...

like your 96!....I wish mine was a 5 speed. the auto tran is "crazy" ......has been since it was new.

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id much rather have started with a GS, ive been looking for 1 for like 3 years with no luck

i converted mine to 5 speed like many other members and its well worth it.

Is the engine loosing oil? I bet the oil rings are shot to have the valves burn so fast.

Lower temp. thermostat will decrease the block temps, but will not do much for the head since it is oil cooled only. It is still a good idea to replace the thermostat with 180 deg one. It will regulate between 180 and 190 deg unlike the stock which is 190 deg.

Clean the EGR, its a 20 min job

Hmmm "oil cooled only."?

Yeah, thanks. I had just woken up when i typed it.

its fixed now

it is loosing oil, and it shouldn't be from the valve stems.....i've put new stem seals every time. i just didn't relate worn rings to burned valves. thanks for the advise!

i have cleaned the EGR, but not sure how to know if it working other than by substitution......and not sure in my case if it is stuck open or closed is more detrimental to the valves..

Clean the EGR with seafoam or a thing called Chemitool B12 or some like that. I get that second one at Walmart for $3. Use an old toothbrush to reach inside the ports

For the EGR to cause any fault it has to be stuck open. That means all the exhaust gasses routed thru it go to the intake manifold and into the combustion chamber. But it cant be that much anyway, so most likely the oil rings are shot.

You can buy pistons and rings for like $100 off ebay and do the work yourself, its very easy. You dont even have to remove the engine, just pop the oil pan open and undo the rod bolts. You'll need basic tools like cylinder hone tool and piston ring compressor, you can find both at harbor freight for around $30. You'll need a tube of RTV to seal the oil pan, new HG, new rod bearings.

Shoot your 90% there only another easy hour and the motor is out on a stand. The fuel system can be the issue as well get injectors cleaned and tested, fuel pressure checked and replace fuel filter and front O2 sensor. Was there something catastrophic at 120k like timing belt? Belt breakage seems to cause more harm to exhaust valves. It could be that minute damage was done at that point and it is just taking its time to really show. I would definitely get the head refreshed with every valve checked including seats, installed height and stem.

....with my 94 GS engine being 4G64 DOHC, am I using the same rings and valves as the 4G64 SOHC?
is it the oil blow-by or weaker compression or both that affects the exhaust valve life?

appreciate your advise on this....i hope i can return the favor to the group in the future..

The rings are the same, valves are not. The DOHC head has bigger valves.

Compression rings and oil rings are different. You can still have compression and burn oil cuz the oil rings are shot. That was my case before the rebuild - I had 190 across the board, but it was burning oil and all my exhaust valves were burnt on my SOHC head.

no...never had anything catastrophic...( i was going to say it was always babied as a family car, but wife could have been giving it hell all along....the sob would move!)...at 120K, I was in the mtns. climbing a steep grade....i'm on the flats so now it happens when i'm punching the auto to pass...i had thought the valve was ready to chip and that just finished it off...

even though i've used two shops and two sources for valves, i didn't know if the shops used specific valve / seat data on the DOHC ....i made a point to advise them

If you got 250k out of the original auto you are doing something right and deserve a medal!

yes...everything under the hood is pretty much original...except of course the E valves

i will check the fuel components and as mentioned above...the cat may be restricted some.........yeah, it's orig....

i would replace the cat...something i need to do to my GS.

thanks.... never seen so much info on the GS.....i've been used to the exact opposite..

welcome to the GS Club...lol. never had burned valves on my GS but I can tell my cat is failing...its OEM but the car has been sitting for years. it got to go.

There can be several factors that can cause you valves to burn...replacing the valves only fixes the burnt valve issue, but the underlining issue hasn't be resolved why you had another burnt valve.

Incorrect timing can cause valves to burn like if the cam holds the valve slightly open. The valve head does not transfer its heat to the MUCH cooler valve seat area in the cylinder head. The valve burns.

If the car is N/A..having staight piping or eliminating converters will cause burnt exhaust valves due to lack of backpressure which helps the cooling cycle.

As what MKO said..a non effecient or an inoperative EGR system, which by design lowers the combustion chamber temperature , can contribute to valves burning.

Bad fuel/air mixtures also can cause the issue too either from bad FPR, bad injector, etc

Insufficient cooling system also can contribute also as the cylinder head isn't cooling effeciently causing overheating (Even though it may not register enough on the dash) to cause a burnt valve.

As you can see there are different factors...hope this helps.

thanks for the tips....no, i haven't found the root cause..
i was thinking wrong about the cat / backpressure....I thought that if i eliminated the cat... it would help cool the exhaust at the header / valves etc...

the EGR is suspect.....it and the path-ways were completely blocked one of the several times i went through this..

i am running rich per the smut on the back bumper.....i need to get an OBD I tool and find out what's going on.

really appreciate the help!......gives me something to work with..

For sure check that AFR. pre-ignition could be at fault as well.

thanks!......i don't hear obvious pinging, but i guess it doesn' have to be audible to be detrimental..

i've been running prem octane........but, would a lower octane be better or worse for this issue?

lower octane would be worse. The GS has a knock sensor and automatically pulls timing when knock is present.

With that said, if you are still on your original knock sensor... I could just about guarantee that its past its prime or worse.
You should consider replacing it with a new 2g eclipse one(same) when you get a chance.

I'm not saying that this is the source or even related to your problem. Just something to consider.

Also, always run premium fuel in your GS. You will lose both power and mileage if you reduce the octane.


Have you ever replaced your fuel filter? Have you routinely run fuel system cleaner?

....yes, original knock sensor, and fuel filter. its amazing it runs at all !!

i've got plenty to do.......thanks!