Head gasket or what?


#1

I need some advice before I tear into this engine to try and find the issue. My truck is a 1988 Nissan D21 King Cab Z2.4i 5 spd and currently has 267,000 miles.



The truck does not want to idle when cold, it will continually stall. Once warmed up the idle fluctuates constantly from 300 to 1300 rpms. Another issue that may be from the same problem or perhaps not is when I park the truck in my driveway (which is a 45 degree angle) the truck will try to roll (trans is in 1st parking brake is off because it is currently broken).



I figured it to be a head gasket, but the results of the compression test is within spec. 1-138 2-141 3-149 4-151. My service manual lists min. psi 125 max. 173 psi. The first 2 cyl. are a little low but all fall withing spec. The test was done with the throttle blocked open and I also added a little oil to each cyl. on a 2nd test to see if the psi increased which it didn’t.



Is it possible the head gasket is leaking between cylinders, but a compression test come out good? I’m not burning or leaking any coolant or oil so I would imagine it is just leaking air during the compression stroke. I don’t mind tearing the head off, but don’t want to needlessly. Is it possible that the truck rolls just because it weighs 5500lbs and has such a small motor? Sorry for the length, but I figured all the useful details possible was best. Thanks for any help.


#2

I don’t think this is a head gasket problem. You need to perform a cylinder leak down test to confirm it. Pressurize each cylinder on TDC look for air bubbles in the radiator, if you don’t see any then it isn’t a head gasket problem.


#3

The truck weighs 5500lbs, parked on a 45 degree driveway and has no parking brake and you’re wondering why it rolls. Well, D’oh.


#4

I don’t think that truck can weigh 5500 lbs. My '70 3/4 ton Chevy weighs 5500 pounds.

I agree with willey that you don’t have a headgasket problem. Is this rig fuel injected?


#5

That steep an angle with lowered compression could definitely allow it to roll backwards. Even good compression will probably not hold it at that steep an angle.

The erratic idle could be anything, but one would suspect a vacuum leak or a faulty Idle Air Control valve. The idle can also be affected the manifold vacuum, which is affected by engine compression.

In your case it sounds like the lowered compression is due to cylinder head valve problems since you state the compression PSI did not increase much while performing a wet compression test.

The next part is that I think this truck has solid lifters with adjustable lash and your problem is not head gasket related. The lash should be checked every 15k miles. If not, the lash can tighten up and over time the valves will not seat properly and the exhaust valves will burn.
If this is the problem you can probably expect to perform a complete valve job and replace a number of valves.

Compressed air can be applied to a cylinder that is on Top Dead Center of the compression stroke and then listen for hissing air at the intake or the tailpipe. That can give you an indication of how badly valves are leaking.

Should I ask if the valve adjustment has ever been checked? If not, ouch; and it’s too late to worry about it now.
:slight_smile:


#6

Thanks for the replies. I’m going to check the valve clearance tomorrow since I’m tearing off the throttle body and gasket and replacing gaskets and sensors to try and get it to idle properly. If the valve clearance is off couldn’t I try adjusting it then and see if it helps. I actually wasn’t aware this truck had adjustable valves, since I haven’t had to tear the valve cover off since I bought it. The truck actually does weigh in at about 5500lbs. It weighs 4800lbs by itself. It’s an off road rig so it has a winches front and rear, brush guard, roll bar and a tool box full of equipment. So it’s pretty beefy weight wise. My 2 other cars that are manual trans can hold my driveway just fine and they are 4 cyl. so I figured this truck should be able to hold, but the cars also weigh half as much. I’ll post what I found out with the valves. Thanks.


#7

There was a post on use of the parking brake yesterday. Engine compression is not supposed to be used to hold your truck in place. Any manufacturer who would even suggest this would face a massive class action lawsuit. On the level you can get away with just putting the vehicle in low or reverse, and not apply the parking brake. The Park position on an automatic is designed as a temporary hold on the level. Even so, the owner’s manual tells you to apply the parking brake. Many of us are lazy, and don’t use it. If you live in a cold climate, you may not want to due to freeze-up danger. In such a case, block the wheels.

If your comprssion is OK, and there is not more than 15% diffference between the best and worst cylinder, leave the engine alone, especially if you do not see any leaks.