If it`s not the valvelifters...

[Cat 920 Sweden]

Hello!
I have a -91 Chevy 2500 with a 6,2l diesel that blows in the intake. I changed the valvelifters on that cyl. but still the same problem.
The engine idles fine but when i raise rpm a bit it starts blowing.
Has anyone had this problem?

[Robyn]

What sort of Blowing??
Is this a harsh popping sound coming from only one cylinder??
You may have a bad valve spring on the intake in question or even some carbon stuck in the valve.

What happened as this condition progressed???
High mile engine??

A weak spring will cause the valve to hang a bit and the lifter will pump up and then hold the valve open slightly and cause the compression to "pop" back into the intake.

Let us know more

Robyn

[sidehackbob]

I gotta vote for the valve springs.

I had a big block chev that liked to break springs where I couldn't see the break. The inner spring would let it idle great, as soon as I revved it it missed. Changed out 7 springs one at a time before I finally replaced the set.

Can you get one of those torque wrench type spring compressers on the rockers of a 6.5?


This is when I hate glue on valve covers.....

[Cat 920 Sweden]

This happened when i was just cruising slowly, thats why i am confused...
The engine has about 350000 kilometers, what`s that in miles?. I don`t think it has had any major repairs.
It`s like Robyn says , the compression pops back into the intake! And just one cylinder.
Sounds like it could be a valve spring.. Can i see if they are broken or should i just try to change them before i do anything else ?

Thanks for your quick answers!

[Robyn]

Remove the intake manifold and run the engine so you can be absolutely sure which cylinder is doing it. (The engine will run fine with it off)

Be careful to keep everything loose like road debris(rocks) that may have found there way onto the engine away from the open intake holes.

I prefer to blow down the engine with compressed air before removing the manifold.

PLEASE be very careful and dont leave any shop rags laying around the engine bay. We had one poor soul who had the engine suck one in.

Once you locate the cylinder that giving issue you can decide what needs to be done.

With the valve cover off you can look at the valve in question.

Most likely if the intake spring is to blame you will notice something amiss with the springs coils. Pressing on it when the valve is closed may reveal that it has very low seating pressure.

To replace the spring in the engine is no big deal.
Remove the rocker arm assembly and then the glow plug. With the cylinder at TDC use an air adapter and force compressed air into the cylinder to hold the valve shut and then you can use a special tool to remove the spring.

The tool is designed to hook on the rocker stud so you may have to get creative with the setup that these engines use.

Be sure to block the crank so the engine wont turn with the air on that cylinder.

Without the cylinder at the top and locked there exists the possibility that if you lose control of the valve or the pressure drops of while you have the spring off, the valve could drop into the cylinder ;0(

If you cant find what the issue is and or the valve cant be held shut with air the only other course is to remove the head to repair the situation.

Hope this helps

Robyn

[Cat 920 Sweden]

I will take a good look at the valve springs, I hope thats the problem!
I have already located which cyl that has the problem (driverside 4th) and the pushrods are straight...

[john8662]

I will take a good look at the valve springs, I hope thats the problem!
I have already located which cyl that has the problem (driverside 4th) and the pushrods are straight...

Cylinder #7.

Squeeze your head in between the cylinder head and the hydroboost stuff and see if you can see the lifters themselves. Look and see if any of the lifters are "collapsed" or of the plunger is mis-aligned (halfway coming out of the retainer clip that holds it INTO the lifter.

You may even want to remove those two lifters and have a look, to make sure the roller is still in good shape.

I also vote to look carefully at the valve springs on that cylinder, luckily there is just two (I pity those fools that have 4 valves per cylinder, cough.. dmax).

Remember there are actually two springs per valve, a spring inside a spring.

J

[Cat 920 Sweden]

I changed the lifters and still have the same problem... The rollers was in good shape.

The truck is lifted 5 inch so it`s too high for my garage , but I will check the valve springs as soon as the temperature outside is a not so cold...:cool:

[More Power]

Well-equipped mechanics sometime use an optical probe (like what they use in spy movies) to snake down the intake runner in the cylinder head, to examine the valve and valve seat.

If that cylinder had ingested a broken glow plug tip, it might have damaged the valve or seat.

Jim

[Cat 920 Sweden]

Thanks for your tips! It was a broken valve spring on the intake valve...:)

[sidehackbob]

Congrats how was the fix outside?

[Cat 920 Sweden]

I just got the new valvesprings and I will try to change them this weekend if it`s not too cold... Not very nice to do this outside in the winter :( !