Renix engine backfiring and running rough

Ok, I just put a junkyard motor in my '90 XJ. It has good compression and I resealed the pan and valve cover and rear main blah blah blah. I went to fire it up for the first time and it starts up BUT it misses and backfires like crazy. I checked the distributor index a dozen times. Tried pointing the rotor before and after the #1 cylinder at TDC compression stroke. I have it right where it should be and it just wont run right. I can't figure this thing out. I am now wondering if maybe the timing chain has possibly skipped a tooth or two and if thats the culprit. I don't know what else to do. Oh, and it has new plugs and cap and rotor. The CPS tests fine. If anyone has any insight please help me out. Thanks alot guys!

Are all the plug wires routed correctly? I know it sounds stupid, but I've done it before. I always seem to forget to check the simple things before I think of major problems.

Jess said:

Are all the plug wires routed correctly? I know it sounds stupid, but I've done it before. I always seem to forget to check the simple things before I think of major problems.

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yeah, no they are just how they are supposed to be I checked them like 10 times trying to figure this out

timing is not set 180* out?

what year engine went into the 90? Do you have all compatible sensors?

Jess said:

timing is not set 180* out?

what year engine went into the 90? Do you have all compatible sensors?

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x2


was it a renix(87-90)engine to renix swap?

Yeah, its an '88 in a '90. I'm at work now but its sitting with its back higher than the front and the tank was bone dry and I only put like 1 gallon of gas in it so it may be running lean and sucking air im gonna try putting more gas in it.

well, it has alot of gas in the tank now and nothing has changed I am really stumped here. I just checked all my grounds and they all test out to have the exact voltage the battery has

ok, so here is another side note: it will sit and idle fine but it has random backfiring and when you try to give it throttle it basically does nothing

Do an ohm test on the CPS, clean the connector. Make sure all the oil and whatnot is washed out of the CPS connector, the CPS puts out a very small signal.
Double check your MAP, the MAP vacuum line and make sure it is plugged into the proper hole on the TB. I've bent a pin on the MAP before and I've actually found them plugged into the wrong connector on the fuel rail harness, hard to do but not impossible, there are some odd connectors in the 90, left overs from the C 101 connector bypass.

8Mud said:

Do an ohm test on the CPS, clean the connector. Make sure all the oil and whatnot is washed out of the CPS connector, the CPS puts out a very small signal.
Double check your MAP, the MAP vacuum line and make sure it is plugged into the proper hole on the TB. I've bent a pin on the MAP before and I've actually found them plugged into the wrong connector on the fuel rail harness, hard to do but not impossible, there are some odd connectors in the 90, left overs from the C 101 connector bypass.

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ok, tried all of that, everything specs out correctly map is correctly (vacuum line goes in the bottom hole on TB since the top one is "blank") nothing changes, not even a little. this thing just sites there and backfires and the longer it runs the worse it gets and when you give it throttle it never raises the RPM's it does pretty much nothing I am so stumped with this thing

When I was having CPS issues it did almost exactly what you describe. Is it possible a connector or the wiring is fubared between the CPS connector and the ECU? I know from experience it is easy to clamp a wire when putting a motor in.
Is it possible the CPS bolts are loose or the CPS got banged during installation, improper pickup distance from the tone ring can also cause the same issues.
When you set up TDC, did you use a compression gage to make sure you were on the compression stroke? Don't assume, do it again, as many times as I've set up TDC I still screw it up on occasion.
Have you done a compression test on the new motor yet?
As a last resort, set up TDC, turn the motor over a few times and set up TDC again, I'm thinking the drive gear for the distributor may have a broken pin.
Most likely to be the CPS, the distributor, ignition, timing related or valves. Though anything that really screws with the mixture may be causing grief. Way too lean or way too rich can make it pop and buck.
Initial timing, turn the motor over until the compression gage registers, align the timing mark to zero. Curve a piece of coat hanger and stick it down the spark plug hole (bend/curve out, towards the valve cover). Back the motor by the harmonic balancer bolt about a quarter of a turn and then turn it to the right until the timing marks align, then go past the timing Zero. What you are trying to do is feel if the piston is all the way up when the timing mark is aligned to zero. Your last movement when aligning the timing mark should be to the right, this takes the play out of the timing chain. If the harmonic balancer bolt unscrews on you, you may have to remove the inspecting plate and turn the motor over at the tone ring starter gears with a screw driver.
An alternate methode, is to put the number one piston top of the compression stroke, timing marks aligned to Zero and then wiggle the intake and exhaust rockers, they should be a little loose and wiggle some. Pulling the valve cover is a pain, but this will sure tell you for sure you have TDC and what kind of shape your timing chain is in.

8Mud said:

When I was having CPS issues it did almost exactly what you describe. Is it possible a connector or the wiring is fubared between the CPS connector and the ECU? I know from experience it is easy to clamp a wire when putting a motor in.
Is it possible the CPS bolts are loose or the CPS got banged during installation, improper pickup distance from the tone ring can also cause the same issues.
When you set up TDC, did you use a compression gage to make sure you were on the compression stroke? Don't assume, do it again, as many times as I've set up TDC I still screw it up on occasion.
Have you done a compression test on the new motor yet?
As a last resort, set up TDC, turn the motor over a few times and set up TDC again, I'm thinking the drive gear for the distributor may have a broken pin.
Most likely to be the CPS, the distributor, ignition, timing related or valves. Though anything that really screws with the mixture may be causing grief. Way too lean or way too rich can make it pop and buck.
Initial timing, turn the motor over until the compression gage registers, align the timing mark to zero. Curve a piece of coat hanger and stick it down the spark plug hole (bend/curve out, towards the valve cover). Back the motor by the harmonic balancer bolt about a quarter of a turn and then turn it to the right until the timing marks align, then go past the timing Zero. What you are trying to do is feel if the piston is all the way up when the timing mark is aligned to zero. Your last movement when aligning the timing mark should be to the right, this takes the play out of the timing chain. If the harmonic balancer bolt unscrews on you, you may have to remove the inspecting plate and turn the motor over at the tone ring starter gears with a screw driver.
An alternate methode, is to put the number one piston top of the compression stroke, timing marks aligned to Zero and then wiggle the intake and exhaust rockers, they should be a little loose and wiggle some. Pulling the valve cover is a pain, but this will sure tell you for sure you have TDC and what kind of shape your timing chain is in.

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well, when I first tried to fire this thing up it had no spark and I changed out the CPS with another one I had because I had broken the old (well it was actually new) one putting in the motor. I am going to check to make sure it is aligned properly and bolted in right first. If that makes no difference then I am going to go through your more detailed procedure for indexing the distributor. Thanks a bunch you have given me 110% in trying to fix this thing. I can't wait to get my TSL's back on top of some rocks heh heh heh. I will let you know if anything works out. OR if it doesn't.

Have you driven it around yet, because maybe there is some buildup on the valves if the engine sat for a very long time. I helped put a junk yard 4.0 in once and it did the same thing until it drove a few miles to clean out all the crap.

IH392 said:

Have you driven it around yet, because maybe there is some buildup on the valves if the engine sat for a very long time. I helped put a junk yard 4.0 in once and it did the same thing until it drove a few miles to clean out all the crap.

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the engine won't even accelerate let alone me driving it around - the way this thing is backfiring I dont even like when I have to start it to test it just to find out its still not right

Try to see how much play the timing chain has by moving the crankshaft back and forth while watching a valve that is moving. There should be little play in the crank before the valve moves.

well, it ended up being the CPS wasnt aligned right - it was cocked and was just off like 1/4" so i got it in place double checked my distributor indexing which was correct and it fired right up thanks everyone for the help and moral support HAHA I am really excited - and this engine sounds really good on top of it all. I actually bought a whole '88 XJ that was badly rotted and needed a fuel pump for $100 and yanked the motor from that so I have lots of extra parts and once I scrap it I will get my money back and have extra stuff - well, thanks again!

well, took it on its first test drive and it runs great

Glad to hear you got her going.. Now go have fun..

scottmcneal said:

Glad to hear you got her going.. Now go have fun..

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Thanks man! Yeah, this thing does some pretty serious rock crawling (probably about as serious as you get with 32's and a bunch of armor LOL). But its alot of times a daily driver as well. I also have a 93 ZJ with a 318 and then the summer vehicle which is a 2004 VW GTI VR that is quite modified.