lilredtruck Posted December 17, 2008 Share Posted December 17, 2008 To preface the thread: here is what needs to happen and has happened. I got an '86 MJ for $30 with a bad V6 - tossed it. Bought a wrecked '90 XJ with a good 4.0L MFI and installed it in the MJ. Started it up yesterday and it runs, but it runs like crap! I tried replacing the map sensor, but no luck. Tried to diagnose the problem with a computer reader, but they weren't talking. Tried to replace the computer, but no luck. Made sure all of the vacuum hoses were plugged, but still no glory. Checked to make sure all of the sensors were plugged in and they were. It runs very rough and is backfiring through the air intake which sounds to me like a timing issue, but there is no adjustment on the distributor. I was told by my friend that the timing is controlled by two pieces of equipment: there's a piece in the distributor another at the back of the engine and these two communicate with each other to keep it running smoothly, but my friend says that it is unlikely that they are bad (need and opinion here from another source - what do you think?). The other part of this story is that I'm trying to get from Montana to Michigan by Christmas and need to resolve this rapidly. It's a three day trip (+/-), so I need to leave by this weekend. HEEEEEEELLLLP! Link to comment Share on other sites More sharing options...
DirtyComanche Posted December 17, 2008 Share Posted December 17, 2008 You used the tranny out of the XJ? The CPS (crank position sensor) is based off the flywheel/flexplate. You have to use a renix (87-90) flywheel/flexplate or it won't work. Otherwise, I'm somewhat at a loss. Link to comment Share on other sites More sharing options...
comancheman Posted December 17, 2008 Share Posted December 17, 2008 Same exact thing happened to me when i swapped in another motor. Turned out to be the CPS. probably a good idea to change it either way Link to comment Share on other sites More sharing options...
JeepcoMJ Posted December 17, 2008 Share Posted December 17, 2008 You used the tranny out of the XJ? The CPS (crank position sensor) is based off the flywheel/flexplate. You have to use a renix (87-90) flywheel/flexplate or it won't work. Otherwise, I'm somewhat at a loss. I'm sure he did...tranny would make sense to be from the same donor. the distributor is not indexed properly...I'm guessing you had it out of the block when you were cleaning/installing the motor or something along those lines. what you need to do, is take out all of the spark plugs, and turn the motor by hand with a breaker bar until you rotate number one cylinder to TDC. then, look at the distributor cap. it has a number 1 on the number one cylinder pole. take a marker, and make a mark on the distributor base that's directly in line with that pole. remove the cap. now, look at the rotor...is it in line with that mark? if it is a little bit (read a very small amount) before that mark, then you are doing fine and it's not the problem (most likely). BUT if it's off by 1/4" or so in either direction, you skipped a tooth during install. remove, turn the rotor back to match the mark, then turn it counter-clockwise by a bit. you'll notice that the drive gear teeth curve. there is also a flat key on the bottom of it which drives the oil pump. now, you're turning the rotor back to a point roughly 3/16" before the mark you made, and you need to take a flathead screwdriver and turn the oil pump back to match (or get close), then drop it back down and bolt it down and it should line up. HOWEVER, this is a 50/50 shot. you can set it to TDC on number 1 or number 6 I believe...so take careful note of the current position of the rotor, and remember if it is set on number 6 or number one, and should it be, then simply keep it going that direction and do this same procedure, simply pointing to number 6 Link to comment Share on other sites More sharing options...
HOrnbrod Posted December 17, 2008 Share Posted December 17, 2008 I sure liked it better in the old days when the flat key for the oil pump was offset so the dist could only go in one way......... :D Link to comment Share on other sites More sharing options...
mvusse Posted December 18, 2008 Share Posted December 18, 2008 How do you know the engine is good. Did you hear/see it run, or are you taking the sellers word? Link to comment Share on other sites More sharing options...
lilredtruck Posted December 18, 2008 Author Share Posted December 18, 2008 How do you know the engine is good. Did you hear/see it run, or are you taking the sellers word? The guy that I bought it from is a good friend of the guy who's helping me with the install. He installed this engine in the Cherokee about a year before it got wrecked. They started it before it was taken out of the XJ. Link to comment Share on other sites More sharing options...
Wildman Posted December 18, 2008 Share Posted December 18, 2008 I was told by my friend that the timing is controlled by two pieces of equipment: there's a piece in the distributor another at the back of the engine and these two communicate with each other to keep it running smoothly, Not quite, the CPS is in the rear of the engine, on the flywheel / Flex plate to be more precise, and the Cam Shaft Position Sensor is in the base of the Distributor. The CPS would give you a No Start at all. The Cam SPS would give you a rough running as you wrote. Jeepco gave you a detail expiation on how to check the distributor, which is the only way to really check the timing on the 4.0. You still can get an idea with a timing light on the crank marks to see if you close, but not any adjustments. Little more details of what you changed over :dunno: Like, everything??? Did you change all the wiring from the '90 XJ to the MJ?? Including the ECM??? Did you install a in tank fuel pump, for the 4.0 and not the mechanical pump on the side of the 2.8?? Oh......and forget the diagnose reader, it will not work on the '90 (Renix) Fresh "boot" on every start, no stored codes. Here's a link for some info on the sensors, and you can find some other info on the main page to trouble shoot your problems - http://www.lunghd.com/Tech_Articles/Eng ... ostics.htm And......to get you from point A to point B before Xmas :dunno: Link to comment Share on other sites More sharing options...
lilredtruck Posted December 20, 2008 Author Share Posted December 20, 2008 Well, I tried a new computer thinking that maybe that was the problem, but it still runs bad (now I have 3 of the damn things). I also tried checking the timing marks at the timing chains, and everything seems to be okay there. Even the distributor seems to be in the correct position. There is a problem which I haven't mentioned to you fellas, and I'm hoping that one of you may know. I've got a dead short in the computer harness; I've not been able to find it in the wiring, but my test light shows a dead short at the battery. I know it's not in the rest of the harness, because we've already disconnected the bulkhead connector and it's still there. Anybody got an idea about this one? Is it supposed to have constant power to the computer which would read through to something? Link to comment Share on other sites More sharing options...
lilredtruck Posted December 20, 2008 Author Share Posted December 20, 2008 The Cam SPS would give you a rough running as you wrote.Little more details of what you changed over :dunno: Like, everything??? Did you change all the wiring from the '90 XJ to the MJ?? Including the ECM??? Did you install a in tank fuel pump, for the 4.0 and not the mechanical pump on the side of the 2.8?? Everything was changed over to the '90 XJ. I took the fuel pump out of an old Explorer and hooked it directly to the fuel pump relay. It's a good strong puller pump and takes the pump out of the gas tank (which I prefer anyway). I also tried to check the timing using the timing mark on the harmonic balancer, but I had a weird thing happen: the timing light was blinking all over the place - not rhythmically like it should. I had the pickup on the #1 spark plug wire, but it was blinking haphazardly. Any ideas about that one? Link to comment Share on other sites More sharing options...
HKB3 Posted December 21, 2008 Share Posted December 21, 2008 are your plug wires on in the correct firing order? I would try to pinpoint the short you mentioned Link to comment Share on other sites More sharing options...
Wildman Posted December 21, 2008 Share Posted December 21, 2008 :hmm: If you used a ford fuel pump, what kind of pressure do you have at the fuel rail?? It should be 31psi with the vacuum hose connected to the pressure regulator, and 39psi with the vacuum disconnected. If you pushing way too much pressure, I could see that being a problem. but I had a weird thing happen: the timing light was blinking all over the place - not rhythmically like it should. Yea, that sound like something that shouldn't be happing :hmm: Check this link for the cam sensor- http://www.lunghd.com/Tech_Articles/Eng ... Stator.htm The only other thing that pops into my head is.........how's the Coil??? And did you double check the grounds........all the grounds?? This could lead you back to where you said you have a "dead short" in the computer harness. It's back feeding thru the harness, and not grounded. I would check the harness to see if there is any "pinched" wires that happen when you did the motor swapping. I know if you have a broken line, you can use a tracer to help find that, I don't know if the tracer will find a dead short :dunno: Link to comment Share on other sites More sharing options...
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