Please help rough idle and overheat

[91 black xlt]

I have a 91 xlt with 141,000 miles. I recently changed the spark plugs and wires. This made the truck run a lot better. I usually have to add water to the radiator every 3 weeks or so. I can’t figure out where the water is going? I took a road trip, I only got 100 miles into the trip and the truck over heated. I made sure that the radiator was full with water and Preston. So I let the truck cool down, I then flushed the radiator and added new Preston and water. I proceeded only to get about 5 more miles down the road and it started to overheat again. This time I heard a loud bang which was preceded by several popping sounds in succession with the engine. So I pulled over again and lifted the hood to discover that one of the spark plugs had exploded into 2 pieces and was on fire. I put it out and waited for AAA to come. The problem I am having now is that I replaced the plug when I got the truck home and started the engine and its running very rough. So I replaced all the wires and plugs again and still the same thing. The engine is idling rough. I don’t know what to do now.

Please help
Bari

 

[mrboyle]

I hate to breake the bad news to you, but it sounds like you have a blown head gasket, or worse. When the plug broke it is possible one of the peices fell into the cylinder and either busted a hole in the piston or got itself stuck in the exhaust valve. The first thing you need to do is a compression test to determine if you still have good compression in the cylinder. If it fails a compression test its time to remove the heads and see what happened.

 

[91 black xlt]

thats what 1 of my friends said it also smokes around the header. how much can i look 2 spend on a compression test

 

[mrboyle]

Due to the differences in labor rates I can't give you an exact price, but it shouldn't take more than one hour to do the work. Also, if you have the test done and it turns out that you do need to remove the heads, a lot of shops won't cahrge for the initial tests.

 

[Maniak]

If you can change the plugs, you can do a compression test (a compression gauge that screws into the spark plug hole isn't that expensive.. and is good to have anyway)..

All you have to do is pull all the plugs/wires (you are supposed to do it with all the plugs out).. and plug the guage into a spark plug hole.. turn the motor over about 5 turns.. then check the compression.. The exact number isn't "that" important.. but if the lowest number is <=110 your motor is getting tired..

Once you have tested and written down each number you want to check the difference from the highest to the lowest number.. If I remember correctly you should have 10% or so difference max.. much more than that and you will have rough running lack of power motor..

If you have a head or head gasket problem you will have low (e.g. we had 40psi on the dead cylinder on our '92) compression on 1 or multiple cylinders..

You can check to see if it is rings that are cuasing the problem by putting in a little oil (just squirt some in the spark plug hole) and then do the test again.. If it comes way up you have a problem with rings.. not heads.. (or a hole in the top of the piston)

If it looks like heads or head gasket and you are very very very very lucky (not likely), you can just change the head gasket and you will be ok for a while.. BUT with the amount of work it takes to pull the heads to change the gasket you might as well get rebuilt heads.. Depending on how bad your old heads are you should be able to get a set for between $220 (need to be good heads, no cracks which isn't likely) up to $700 for a pair if your heads are toast.

When we did the heads on our '92 both heads were cracked, but one was repairable.. so we got $100ish credit for the one. Total cost was close to $600 for the heads.. plus another $100ish for all the gaskets.. The expensive part is going to be labor.. Your looking at 13 hours of work.

At least with compression test you know where to start.. If the compression test shows everything is fine then you dodged one bullet.. and know to look elseware.

~Mark

 

[91 black xlt]

Thanks Mark. I got it back from the shop today. the number 3 cylinder shows no compression. the mechanic believed it was the head gasket.

 

[Glenn Biggs]

Make sure they do a good inspection of the heads. They are prone to crack in between the valves.