new here,92 misses and then doesn't.#5 dead when missing

Hi here it is, start up cold let idle until the miss goes away or drive it to work with it missing and then runs fine after work. I am stumped, has any one out there heard of this? I have to date put in plugs, wires, rotor,crank ps and 3 inch cat back which makes it run stronger on 5 cylinders but does not fix the miss. please anyone?

How is the injector on #5? How do you know it is #5 that is dead?

DieselSJ said:

How is the injector on #5? How do you know it is #5 that is dead?

Click to expand...

Agree. What tests have you done to determine which cylinder is missing? If you have tested, is it missing spark or fuel?

I have a thermomelt stick that I touch on the header tubes and it melts different on #5 and the ruff idle doesn't change when I pull the plug wire. If it is the injector, how do I check it? thanks

Ok, it should "click" each time the PCM sends the ground impulse and the injector fires.

easy (read free) check--
swap injector#5 with injector#1 and drive her. If the misfire follows the injector to cylinder #1, the injector is bad. If the misfire remains on cylinder #5, you can/should start troubleshooting that location.

I will do that and I also installed an injector from the wrecking yard that I had a friend ultrasound clean for me. I know that isn't like what the auto store does but I don't want to poke around in the dark. thanks

*Didn't see any mention of distributor cap

*Swapping injectors is a great idea

*After that, it's time to check compression on that (and all) cylinders

Double post due to computer problem. Sorry.

Yes there is a new cap and if you let it idle long enough it will stop missing. It has 176000 miles and when it doesn't miss it runs really strong. The plugs look good and it doesn't use oil. I bought the jeep a couple years ago so that I could save my dodge cummins from the liquid rust that they spray on the roads here in the winter and consequently fell in love with the little car. thanks for the hellp

Rather than swap whole injectors, swap adjacent injector connections, and you can then see if the problem follows the injector or the harness. Although injector firing is timed, it's not so critical and will work well enough for the test.

Assuming you've already eliminated spark problems... if an injector is firing correctly, when you pull the connector at idle, the engine will bog slightly, then recover. If the injector is not firing, you'll see no change. Do the test with connectors normal, then shut down, swap connector of whatever injector shows bad with its nearest neighbor, and try again.

Have you tested vacuum? It's a good way to narrow things down. A bad injector won't show much on vacuum, but of course mechanical problems will, and so, usually, will spark problems.

edit to add: I had similar problems with a 93 that had a bad splice in the injector harness, but that one got worse when it heated up. Still, it may be worthwile to get an ohmmeter and check for resistance in the harness wires. Look especially at the +12 volt supply to the injectors, which is sourced at the distributor, and sent to all injectors via splices (the injectors are switched by the PCM on the negative side).

cool, I will do that, thanks

well the trick of switching the injector leads around worked great. it turns out that it is in the wire. So I went over to my local auto sparky, and of course the jeep started running great, and he thought I could check with a test light the next time it was missing. Here is my question, can you tap into another injector lead for your power or do you have to go all the way back to the splice? And, what do you do if it is the ground back to the pcm? thanks to all of you. you can't believe how far I have come. cheers

I would repair the harness.

Once you start chopping up any electrical circuit tears are soon to follow, but the engine management circuit is one you should treat with respect.

Just IMHO.

When I had this on the 93, I opened up the harness and bypassed the bad splice with a piece of wire, which I soldered in. I didn't even bother to remove the bad splice. I wrapped it back up nicely, and put the bundle back together, and it was trouble free for years after that. I don't see any problem with repairing the harness as long as you are careful, but you will have to make sure that you don't open anything up to added corrosion and stress. What to do if it's the line from the PCM really depends on where the problem is. Hope that it's not the PCM itself.

Thats good advise, but I have been running around without the plastic harness cover off so that I am ready to check and see if it is the power or the ground, and it continues to run well. How far back do I have to go to get to the factory splice and do you think the cover would have any thing to do with this problem? would it be foolish to swap in a different harness? thanks

joelpop1 said:

Thats good advise, but I have been running around without the plastic harness cover off so that I am ready to check and see if it is the power or the ground, and it continues to run well. How far back do I have to go to get to the factory splice and do you think the cover would have any thing to do with this problem? would it be foolish to swap in a different harness? thanks

Click to expand...

First you need to be sure which wire has the problem. If you have access to another harness, I suppose that's a good solution, but if not, I'd go ahead and repair. If the problem is in the common +12 volt circuit, you don't have to go back to the splice, which may be buried deep in the harness. You can just solder in a bridge between the affected injector's wire and that of any known good injector. The branch lines are the same gauge wire as the trunk, so it doesn't matter where you tap power off. Peel a little insulation off the wires, splice on the bridge and solder it, tape it well and cover it all up again. You can leave the bad splice right where it is and it won't hurt a thing.

This is the best news yet, being able to splice into a common 12v just made my day. Have a great weekend.

update: it was not the power side of #5 injector but the switching side and all it took to fix was to clean the harness plug and the computer side of the plugin and make sure the grey and black stripe wire was pushed all the way in. Moral of the story? do your simple stuff all your simple stuff first. Thank you all who lent me your thoughts and knowledge because this was a long journey for me. cheers