burnt valves?

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haydn28

Member +
im getting a miss fire ONLY on idle , no white smoke , no backpressure , pistons are forged with trd head gasket and car was tuned to 20psi on 95RON , experienced some slight audible knock last night and car drove well home , only this morning i noticed a change in idle , plugs , wires and injectors are good.
compression test : 150 psi on 1 and 4, 95psi on 2 and 3 ,

ive never burnt valves , it seems like the most likely problem , anyone out there experienced this ?
 
350ep70gr

350ep70gr

Member +
Sounds like hg fail between cylinders if not something more serius. Valves can be burnt quickly if wrong clearence is selected. So drop the rocker cover off and mesure the gap on the suspected valves to rule that out fast.Or you can drop the mani ..put the suspected cylinder on intake stroke and blow some pressure air throu the inlet.If you get some air coming out from the exhaust side then your valves are bend or burned wich i highly dout to happen to two cylinders at the same time.You would have notice a drop in performance first.


Chris
 
H

haydn28

Member +
350ep70gr said:
Sounds like hg fail between cylinders if not something more serius. Valves can be burnt quickly if wrong clearence is selected. So drop the rocker cover off and mesure the gap on the suspected valves to rule that out fast.Or you can drop the mani ..put the suspected cylinder on intake stroke and blow some pressure air throu the inlet.If you get some air coming out from the exhaust side then your valves are bend or burned wich i highly dout to happen to two cylinders at the same time.You would have notice a drop in performance first.


Chris
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i was thinking the same thing (blown gasket between cylinders)....but I've never had a trd gasket blown before ......Im pretty sure its not pistons , cause no white smoke at all, no back pressure out the breather , it runs smoothly once u rev it its only on idle Im getting the mis fire ....

the engine has cruse cams on it im not sure about the clearance.

will take off the head and check
 
350ep70gr

350ep70gr

Member +
Have you skimed both surface before you installed the gasket?Metal gaskets exept copper require pefrect surfaces to seal strong.
I dout the engine with two that low comp cylinders perform well. Should be a major power loss for sure.May sound ok tho..but the performance should be serius reduced.
Before taking out the head..drop the rocker cover and mesure those 8 valves.
Chris
 
H

haydn28

Member +
350ep70gr said:
Have you skimed both surface before you installed the gasket?Metal gaskets exept copper require pefrect surfaces to seal strong.
I dout the engine with two that low comp cylinders perform well. Should be a major power loss for sure.May sound ok tho..but the performance should be serius reduced.
Before taking out the head..drop the rocker cover and mesure those 8 valves.
Chris
Click to expand...

surface was skimmed by a machine shop , the power is down for sure though , will check the surface again weekend for sure
 
T

TheStarletGT

Member +
Looks like you broke pistons.

Reason is that there is no way if you have a blown head gasket or burned valves, 2 cylinders have low compression but close in regards to figures.


if you have a blown headgasket between cylinders, the comp should be 50psi each, thats hwta i experienced with another car.

90psi and 75psi shows major issues.
 
G

GP82

Member +
Have you opened her up and checked to confirm it is burnt exhaust valves?

The tune on the car can play a big part in this if it is not mechanical\headgasket or clearence issue or even a bottleneck in your waste system. It could be the car is running more then 10% lean from the point it makes best power and therefore exhaust valves have taken the damage from the excessive heat caused from a lean mixture. Do you have a EGT gauage to monitor waste temps?
 
H

haydn28

Member +
GP82 said:
Have you opened her up and checked to confirm it is burnt exhaust valves?

The tune on the car can play a big part in this if it is not mechanical\headgasket or clearence issue or even a bottleneck in your waste system. It could be the car is running more then 10% lean from the point it makes best power and therefore exhaust valves have taken the damage from the excessive heat caused from a lean mixture. Do you have a EGT gauage to monitor waste temps?
Click to expand...


the car was perfectly tuned to 300whp on 95 ron , but i suspect the quality of gas i fill up on was POOR . and i backed off instantly once it knocked

"TheStarletGT"
Looks like you broke pistons.

Reason is that there is no way if you have a blown head gasket or burned valves, 2 cylinders have low compression but close in regards to figures.


if you have a blown headgasket between cylinders, the comp should be 50psi each, thats hwta i experienced with another car.

90psi and 75psi shows major issues.
Click to expand...

cylinders 2 and 3 are both 95psi exactly,
reason i asked about burnt valves are that not a drop of oil is coming out the breather no white smoke and back pressure from the crankcase is normal as ever . so i would not think pistons likely , i seen and had many engines (stock pistons)break ring lands over 10 years of 4e/5e racing and know the symptoms only too well. forged can take serious knock and not break ....
the head was torqued to 50psi with arp head bolts and trd gasket , and only after the slight knock it mis fired ...:confused: i wondered if anyone had burnt exhaust valves before..

appreciate all the responses to this thread and will take down the head saturday :cool:
 
350ep70gr

350ep70gr

Member +
Happy you found the prob! Add a set of fresh headbolts aswell and make sure both surfaces are death straight.

Chris
 
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