Rough idle after motor swap...
 

I have a 1994 xj and just swapped in a 93 grand cherokee motor. Only used the head, block, distributer...most of the other parts were taken off the xj block and installed on the grand cherokee block. Changed out plugs and wires with new after I realized it was rough. The idle RPMs don't appear to fluctuate with the hiccup.

I didn't touch the distributer. So, I don't know if that's my problem. I'm about to check spark. I suspect it's fine. New plugs and all. Could make plug wires spark trying to reach plugs already. Might take them out and check in the tester.

Bosch Platinum gapped at .040 were installed. Autozone wires. Twice. And yes, firing order is right. However, I hope I have number one right. I'll check that now.

Checked fuel rail. Nothing but a drip maybe came out when a screw driver was pressed to valve. Primed a few times with the key and it squirted out like normal a few inches. Started and did it again with it running ( I know...at this point I don't care if it catches on fire...lol) and it shot out over the fender. So, I don't think I have a problem with fuel pressure. However, I don't think it should leak down. I just double checked my findings and there was no fuel coming out after sitting for 45 minutes to an hour again (screw driver in valve ).

Any thoughts?

***Edit***

I've pressure tested the coolant system and all is well. After I fixed a cracked hose, it's held right at 13psi for about 25 minutes. It's good enough at least.

Replaced the fuel pressure regulator with one left over from an old motor that was working fine. This didn't solve the issue.

Found a few cracked vacuum boots. Took them off and wrapped them with electrical tape for now. Nothing.

theres a check valve that prevents pressure decrease. fixing/installing one is pretty straightforward.

Thanks. Is it on the front side of the fuel rail with a vacuum line coming off the back? Think this could cause the roughness?

Try changing out the crappy over rated Bosch plugs and put either Champion copper or NGK copper plugs gaped at .035. I tried the Bosch plugs and it ran really bad and rough. I changed them out the very next day and put the Champion plugs and it ran perfectly smooth. I don't know if that will cure you problem? But it made a big difference in my 98.

This is the second set I've installed. While I don't disagree these plugs suck...I'm not sure it's my issue. The old plugs were gapped at .035. And they weren't really old...bought about 5 months ago and had about 10 miles and an hour or two of run time on them.

Couldn't tell ya.. I've got a 99, so its a different set up. I'm doing some research now.. It is more likely somewhere underneath, closer to tank.

edit: Still not sure about item you described, a pic would help tons, but either way try adding a check valve close to tank. Pretty sure its 3/8" ID, but thats not gospel. This should help get rid of pressure drop, don't know if it will fix the rough idle..

This is the thing I was talking about.

I swapped it with a known working unit. Still isles rough. I could drive it, but, jumpy and shakes the jeep. Not quite right. Almost too much to be a timing issue.

I wouldn't think the distributer between a cherokee and grand cherokee would be different? Maybe I'm wrong. Does anyone know?

Firing order is the same 1-5-3-6-2-4. Maybe the o2 sensor. Its pretty common for them to cause a rough idle, hesitation and pop/backfire at the manifold.

Is there a way to test and see if it's working right? Wouldn't it throw a code if it was malfunctioning? I don't want to throw parts at it until something works. Half the fun is figuring it out. Ha. Plus, I'm on a budget. Need to save a little money for the winch and bull bar to go with my new bumper.

On a side note about codes...would I need to drive it for those to show up? It's I depend a total of about an hour or so and been driven to the end of my street and back. It has a few other issues that are preventing me going far.

Is that electrical tape on the vacuum fitting at the fuel pressure regulator?

Yes, it is. The boot was cracked. It was a quick fix that should seal it for the time being. I've found a few that I repaired like that. It was just temporary to get it running right then I will replace the boots all at once once I can isolate the problem. Think that could be my problem? Tape not sealing well?

I took off the vacuum line running to the canister behind the bumper and blew into it. I can hear another small leak somewhere near the firewall in front of driver seat. No noise was made there. It was dark last night so I didn't have a chance to locate it. I was going to check again tonight when I got home.

Did you test run the donor engine before you pulled it out of the Grand?

See if it idles differently when you unplug the vacuum hose from the FPR than it does with the tape job.

No. It was a JY motor with a 30 day warranty. I took it apart and inspected and replaced all gaskets. The only thing I didn't do was pull the head. Probably should have for piece of mind.