Hard starting and white smoke after head change.

[U12RACE]

I have a '91 GMC Suburban with a 6.2 that I have owned for about 3 years. I use it for daily transportation and for towing my race car. It has always started and run very well until it developed a coolant leak into #6 cylinder. The problem turned out to be a cracked head. Since there were several cracks in both heads, I decided to replace both heads with rebuilt heads.
The company that supplied the heads (through a local Advance Auto Parts) said that many of the 6.2 heads required so much milling to get rid of the surface cracks that they supplied steel shims with the heads to go under the head gaskets to restore proper compression and valve train geometry.
When I recieved the heads, I measured them from the top of each head (where the valve cover sits) to the machined surface. They were only .010 shorter than the old heads. However, the shims that were supplied with the heads measured .030. After installing the heads, I have a very hard time getting the engine started. It seems to crank over much faster than before. When it does begin to "fire", it seems to just start on 2 or 3 cylinders with a lot of white smoke which lessens as more cylinders "kick in". After it warms up and I start to drive it, it smokes (white) for about a half mile or so and then quits smoking and runs fine. When I restart hot, it starts instantly with just a little white smoke and runs fine. Leaving the engine heater plugged is is about the only way I can get it started at all when cold - even in the garage at 30 degree temperatures.
I ran a compression check after warming the engine up and all cylinders measured between 355 and 360. Is this too low? Unfortunately, I did not have a diesel compression gauge before changing the heads.
I have replaced the lift pump and fuel filter. I have also put clear tubing on the injector pump return line and have seen no air bubbles.
Any help will be greatly appreciated. - Kyle

[DmaxMaverick]

Sounds like timing, valvetrain, and/or glow plugs or glow plug system.

Your compression test was well within the normal range. Also, milling the heads does not effect the compression ratio, but does the valvetrain geometry. Correcting the valvetrain will decrease the C/R, but not significantly. Having it milled .010", then correcting it to .030" will overcompensate the valvetrain. You now have .020" more valvetrain clearance (lash), and less valve extension. .020" valve extension change isn't much, but the compounded rocker ratio will be nearly double that, which will effect breathing ability. The hydraulic lifters should compensate for this, but can't compensate beyond its extended hydraulic maximum. You'll have to manually check the valve lash to know if this is the case. If the lash is excessive, you have 3 options (actually 4, but ignoring it doesn't sound like a real option for you). Replace the rockers with aftermarket adjustable. Replace the pushrods with a compensated size. Or, install the correct head gasket. (in the order of my preference)

If using the block heater helps, it could be helping with any of the above possible problems. I suggest checking the glow plug system, first. Then the valve lash. If the timing set (chain/sprockets) are original, they could be a contributing factor to your problem. An injection pump timing adjustment can help this, depending on how worn the components may be. The pump is driven by the timing set, and will track timing set wear.

[john8662]

The max you can remove from a cylinder head is approximately .012".

These heads utilize non-adjustable rocker arms which require that the head remanufacturing be great, so clearances in valve recession, as well as overall stem length have to be taken into careful consideration. Usually involves getting the valve recession correct, either with new seats or proper valve, then grinding the valve stems to correct length.

After all this is done, no compensation is needed.

The shims are completely un-necessary, if the above reman (per GM) was done correctly.

The only need for shims or a thicker head gasket is when the block is decked, allowing the piston to protrude further from it's new deck height, which will lead to unwanted piston to valve interference.

Effectively, you've almost lowered the compression ratio by 3 points with .030" thick shims.

This is the reason for the harder starts. It's starting like a worn-out engine, due to the substantially lower compression it's building due to the increased volume in the combustion area, due to the shims.

It's like starting the 18:1 engine in my drag truck on a cold day (less than 50 degrees outside) with standard glow time, not real easy, but possible.

[U12RACE]

Thanks for the input. I have tried advancing the timing about 1/8" each direction with no results. I am going to try cranking up the pump about 1/8 of a turn as a last option before pulling the heads back off. Is it possible to measure the heads to find out how much has been milled off of them? As I said earlier, they measured .010 shorter than the heads I took off when measuring from the top (where the valve cover sits) to the milled surface. However, I don't know if those heads had ever been milled. If it turns out that the rebuilt heads have only been milled .010 and I remove the .030 shims that were supplied with the heads, the rocker shafts would only be ten thousandths closer to the block than with my old heads, right? Would the hydraulic lifters be able to compensate for that amount? If not, does anyone know where to to get adjustable rocker arms for a 6.2?
Sorry about so many questions, but even though I have done complete engine builds on gas engines, this is my first experience with a diesel. - Kyle

[john8662]

Remove the shims.

Yes, the hydraulic lifters will be capable of compensating, no issue there. That's part of the reason why GM accepts up to .012" taken off the heads, still safe.

There are no factory adjustable rockers for these engines. This is why the head rebuilding process needs to be done correctly.

Once you pull the heads off, you have the option of taking one to a local machine shop who rebuilds heads and have them check everything per the specifications in the AERA program/book.

[Robyn]

With a .010" cut on the Block there is an available .010" thicker gasket from felpro to compensate for this.

A drop in compression of 3 points will make these little beasts a tad cranky to start. What is the condition of the injectors. If the injectors are down much on the POP pressure and the spray pattern is getting slopy then this will compound the issue. Also if the cold advance in the IP is getting tired this will further compound the issue.

An 18:1 engine will start oK if all is well and the glow system is up to snuff but, if the CCA is not working right and the squirts are tired you will see poor starting.

Check out the squirts and the IP before spending a lot of $$$$ and time pulling out your hair.



Best

Robyn

[U12RACE]

Thanks for all of the advice. I have removed the shims and the engine is running great. All of the cylinders a within 7 psi. of 400. The starting is much better but still not nearly as good as before. I don't think that my glow system is working as it should. With the temperature below about 40 degrees, I have to use my manual override to hold the glow plugs on longer. If I don't, the glow plugs will come on for about 6 seconds then off about 5 seconds. When they continue to cycle, they only come on for about 1/2 second at a time. If I turn the key off and then back on, the plugs will not come back on. Is this normal? I am thinking about replacing the controller.

[john8662]

Good to hear, compression is excellent.

Yes, your glow system isn't functioning normally, suggest replacing the controller. You should also verify the grounds on the back cylinder head near the firewall (since you had the heads off) on the pass side.

1991 6.2's also have another relay that turns on the controller (based on ECT, more to disable when too hot to need), that sits near the firewall/inner fender on the driver's side of the engine bay, so if the controller isn't it, this could be it too.

J

[U12RACE]

:)Yesterday, I put on a new glow plug controller. This morning, the temperature was 10 degrees and I did not have the block heater plugged in. I turned on the key and as soon as the glow light went out (after about 6 seconds) I turned the key to start and it was almost like turning on a light switch. The engine started in about 1 second and was running smoothly in about 5 seconds. Again I would like to thank everyone for their input. - Kyle