3 cyl

[Pete]

Been working on a truck for about a year....new glavanized chasis etc etc etc. At a point where it is time to fire it up. New plugs, wires, checked/gapped points and everything is on. 1st attempt, huge backfire that completely blew the new muffler apart. Adjusted distributor, got it started and running but only on 3 cylinders. #4 cylinder is guaging at 175lbs and #3 is only pushing 125.....question is: Is this a bad headgasket letting #3 leak into #4 or a bad valve? Thoughts?

Pete

[kevkon]

My first question would be , how exactly did you ascertain that it's only running on 3 cylinders? Second would be, do you mean that one of the cylinders is completely dead?
Compression testers can be very helpful, but they can also be very inaccurate. That's why it's preferable to use a leak down tester. Still, even with a low compression reading the cylinder should fire unless it's not getting a spark or not getting any fuel mixture.

[Pete]

3 cyl

No cylinders are dead. However, #3 is at least 10lbs lower than #1 and #2 and #4 is way above normal. Can tell it's running on 3 cylinders by how rough it's running and by pulling number 3 lead off the plug is doesn't change the idle etc. Can leave the lead off and starts the same. Any other lead off makes a huge difference.

[kevkon]

You checked the spark plug wire? Are you getting a good spark at that plug?

[jp-]

I agree with Kevkon, swap the plugs around and then swap the wires. 9 times out of 10 it is spark related when running on 3 cylinders and not fuel related. Was the engine rebuilt? Was the head machined? On my rebuilt 2.25 I had 145 on all 4 cylinders. Perhaps the piston is slightly taller in that cylinder, or the head was machined incorrectly, or the rings are just really tight. If the plug swap and wire swaps do nothing, try a new distributor cap.

[Pete]

3 cyl

Thanks guys. The engine was not rebuilt. Current owner of the truck I am building it for said the engine ran excellent last time he ran it a couple years ago. Had my cousin (he's been building Rovers for a lifetime) over to help diagnose, he brought a new dist and we used that. Changed plugs, wires etc. I put the old dist back in and ran the same. I just ordered a new dist etc to for piece of mind and I'll try it when it comes in.

[thixon]

Pete,

Is it possible that its 180 out?

[Pete]

3 cyl

If it were me working on it yes....my cousin, no.

[Nium]

Put a vacuum gauge on and see what readings you get. Here's a site on diagnosing with a vacuum gauge, just in case...

http://moodle.student.cnwl.ac.uk/moodledata_shared/cdx%20etextbook/dswmedia/toolsEquip/hpt/measuring/vacgauge.html

With a different dizzy and the same results the valve train may need some investigating or could be as you think and a blown head gasket. A leak down test may be helpful in narrowing down possible suspects.

[bmohan55]

Trick an old mechanic showed me, hold a dollar bill next to the exhaust, if it intermittently gets sucked in then you've got either a blown gasket or sticking valve.

[thixon]

Thanks guys. The engine was not rebuilt. Current owner of the truck I am building it for said the engine ran excellent last time he ran it a couple years ago. Had my cousin (he's been building Rovers for a lifetime) over to help diagnose, he brought a new dist and we used that. Changed plugs, wires etc. I put the old dist back in and ran the same. I just ordered a new dist etc to for piece of mind and I'll try it when it comes in.

Pete,

I'm confused. Are you saying it rant the same with the dist your cousin brought over, or it ran better? I think you mean it ran the same with both that one, and the old one. Its seeming to sound more like a valve issue, but why not pull the head? It won't take long, and chances are you'd find the issue whether its valve related or a blown head gasket.

[Pete]

3 cyl

Correct. It ran equally as bad not matter what I tried....new plugs, wires, dist etc.
I'm going to pull the head either today or tomorrow.

[brucejohn]

Put a vacuum gauge on and see what readings you get. Here's a site on diagnosing with a vacuum gauge, just in case...

http://moodle.student.cnwl.ac.uk/moodledata_shared/cdx%20etextbook/dswmedia/toolsEquip/hpt/measuring/vacgauge.html

I am with Nium. Try a vacuum gauge, once you do you will never go back to diagnosing engine issues without one.

[Pete]

3 cyl update

Pulled the head to find a little oil in between the #3 and #4 cyl. Head gasket looked ok but obviously was leaking between. Had the local machine shop test the head and deck it. Ended up being off .006. I installed the head, did a new compression test and all cylinders tested out at 150lbs. Started the engine and it now running like a top.

Pete