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Damaged my engine, likely causes?

ManNamedJed

NAXJA Forum User
Location
Reno
Today while driving my '93 XJ I floored it get through a yellow light. Had it wound up pretty good and something 'popped' and now it barely runs. It seems to idle pretty good, but give it any gas and it sputters, rattles, coughs and generally barely runs. It really can't get over 2200 rpm.

I checked fuel pressure and its at 35psi.

I'm thinking its likely something in the valvetrain - bent valve, broken rocker, bent pushrod, etc. I'll take my valvecover off tomorrow and see if its anything obvious.

Any other potential culprits? Anything else I should check?

Thanks.
 
A compression test is a good place to start.
 
I would start by checking all of the basics. A popping sound would probably indicate a mechanical failure, so I would skip the electrical and fuel possibilities, as you probably already are. Pull off the valve cover and look under there. Pull off the spark plugs and look at their condition. Look into the spark plug holes with a small bright LED light to see if there is any obvious damage. Whats really neat these days are those $50 snake-scopes that you can hook up to your cellphone. Really neat for running into the spark plug holes and seeing the condition of the valves and piston tops. I would also manually rotate the engine while looking into the spark plug holes and see if all of the pistons are rising and lowering as required. While doing this, pay attention to the firing order and make sure that they are following the correct order. Its a little bit more work, but pulling the oil pan to see of anything is going on under there is probably a good idea at this point, too. Pull off the bellhousing lower shield and check that flexplate and the bolts, also the rear main seal area. Theres lots to check, so leave no stone unturned!
 
I hate to say it but it sounds like you may have thrown a rod. A compression test is first on your list;
 
Thanks. Good advice here.

I did just recently buy a harbor freight inspection scope to look into some plumbing in the house. I'll use it to look in the spark plug holes.

I'm really hoping a for something simple like a broken rocker arm....
 
maybe timing chain hopped a tooth
I would not rule that out, even though this does not seem common with the 4.0.

To the OP, if you own a vacuum gauge, that can provide good data in conjunction with the cranking compression.
 
Is it still making noise? Most mechanical problems keep on making racket.

The RPM limit seems to point to a sensor issue. I've run my 88 without one and again without two cylinders (just curious), no RPM issues. Though it did mess with the balance of the motor and it jumped around a bit.

I'd start simple and look inside the distributor cap. If you know how find TDC 1, check the position of the rotor, should be slightly past the number one pole on the cap. More than few XJ's have lost a tooth on the cam/distributor drive gears. There is a shear pin in the distributor shaft that can break, allowing the rotor to shift and may jam again (for awhile). Pull up on the rotor see what happens, then try twisting it hard both directions, but not so hard that it breaks.
 
Thanks for the replies. I haven't had a chance to dig into it yet (benefit of having multiple cars, you don't HAVE to fix things immediately), I also have a broken pipe in my basement that has my attention right now.

I'll check under the valve cover first, if there isn't anything obvious there, I'll run through the other suggestions here. The way it acts reminds of of when I broke a rocker on my chevy truck, so that's where I'm starting.
 
I would agree with a compression check since its easy. Also, bad motor mounts + WOT could cause stress on the intake boot. May just be a gnarly vacuum leak (which coincides with your description of problems). I wouldn't rule out something simple.
 
I am having almost exactly the same problem, but I will start a new thread on it right now so as not confuse the two, so I invite all to post there ideas there also please. I will add one suggestion for now, check the cat converter to see if broke inside, that would back up the exhaust and cause your issues. Drop by my new thread too as it will give you ideas also.

Here is my thread about my current version of the same problem:

http://www.naxja.org/forum/showthread.php?p=246493967#post246493967

Today while driving my '93 XJ I floored it get through a yellow light. Had it wound up pretty good and something 'popped' and now it barely runs. It seems to idle pretty good, but give it any gas and it sputters, rattles, coughs and generally barely runs. It really can't get over 2200 rpm.

I checked fuel pressure and its at 35psi.

I'm thinking its likely something in the valvetrain - bent valve, broken rocker, bent pushrod, etc. I'll take my valvecover off tomorrow and see if its anything obvious.

Any other potential culprits? Anything else I should check?

Thanks.
 
In case anyone is curious, I pulled the valve cover and there is no obvious damage there. All rockers/springs/pushrods look fine. Its actually amazingly clean and nice looking under there.

So no simple problem. Going to have to dig deeper. Borrowed a compression tester and will see what that says.
 
I would still check the cat converter to if fuel was set off in it and damaged the Cat honeycomb such that it is now clocking the exhaust.
See the link below to my thread and find the vacuum test details by Cruiser54 that tests for leaking valves, good compression and blocked exhaust!!! in a simple 5 minute test using an intake vacuum hose. That tests is the easiest there is.

CPS, spark ICM-HV coil issue, and vacuum leak at the line to the throttle body going to the MAP sensor should also be inspected first. I have seen backfires blow the MAP sensor vacuum line off the throttle body.


I am having almost exactly the same problem, but I will start a new thread on it right now so as not confuse the two, so I invite all to post there ideas there also please. I will add one suggestion for now, check the cat converter to see if broke inside, that would back up the exhaust and cause your issues. Drop by my new thread too as it will give you ideas also.

Here is my thread about my current version of the same problem:

http://www.naxja.org/forum/showthread.php?p=246493967#post246493967
 
Finally had the chance to work on this a bit more. I put my timing light on it, and at idle, it looks to be about 16 degrees advanced at idle, which I believe is about right. It certainly doesn't look like the timing chain skipped. It will idle like this okay for 10-15 seconds, then the timing hops around and the firing gets erratic.

Thoughts on the cause of this? Something in the distributor? Perhaps crank position sensor? What else on these controls timing and ignition?

Thanks!
 
Many possible causes, Start with checking the MAP sensor vacuum line for any signs of soft, brittle tubing, hose, or loose connections first, and make sure you can blow air through the line to the MAP. Any of those cause back fires!!!

Than get a Vacuum gauge on it, they are supper cheap at Harbor freight, and will test many things all at once, like blocked exhaust at the Cat, bad Vacuum at the MAP sensor line.

Disconnect the O2 sensor and see if the problem stops!!!!

Those are the simple fast tests at this stage.
 
What happened to testing the compression?
 
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