Help with 88 Cherokee 4.0-Backfiring issues!!!!

[Jason01]

Transfercase? NP229?

I have a problem with my 88 Cherokee. It is backfiring(intake and exhaust). The engine was just rebuilt, and the head is sealing good. I have done a leak down on the engine, and the only place it is leaking past is the rings, and none past the valves. I have checked the timing(both chain and dist.), the Crank position sensor is good(I have a lab scope), as is the sync sensor. I have put in new plugs, wires, dist cap, rotor, TPS, 02 sensor, knock sensor, Coolant temp sensor, MAP sensor, injectors, Fuel pressure regulator, fuel pump, and the fuel filter.

Basically, what it does, is that it will idle just fine for a min or 2, and then will start to die off, and flare up again, and just about die. When it does this, the vacuume will drop to maybe 1-3in. hg. If you open the throttle at all, it will stumble and die. If you are lucky, It will stumble, and shoot flames out of the TB. I am at my wits end here. PLEASE, some one help me, as I am likely to shoot the jeep!!!!! :evil:

Any help will be appreciated!

 

[jeepin916]

are you sure the timing is ok? where is it set to? im not too familiar with an 88 cherokee but i know on some of the newer engines you have to set the distributor to 0 and the computer will take over the rest, if you have all this thecked and everything is fine it could be the computer not adjusting your timing at higher RPM's

 

[Jason01]

I have double checked the timing. I point it to no. 1 cyl when no. 1 cyl is at TDC, computer controls the rest. It wont even run past 1300 RPM's

 

[jeepin916]

i would starrt checking the computer signals on a scanner then.

 

[Jason01]

Sealing up distributor...do I need a vent?

This cherokee uses AMC's computer setup. I have allready checked with the local dealers, and they all say that they can look at it, but that they dont have the computer to tap into it.

 

[XJNick]

Actually, most pre 1990 Jeep cherokees are equipped with the not so good Bendix/ Renix II computer system. These computer systems do NOT store codes! It also has NO check engine light.

Instead, the computer system can only be hooked up to a "monitoring" computer that will display the current state. Codes or faults are not stored in this system.

It's hard for me to at the moment think of any particular item that could be causing the problems. But as a general tip, don't expect all new computer sensors to work correctly. If there is any hint that the computer system is causing these problems, I'd retest all the major sensors (MAP,CPS,TPS,MAT,CTS, etc.).

-Nick :!:

 

[jeepin916]

is there any way you can have it hooked up on a scope to see the fireing patterns, and all that good stuff? maybe somethings not fireing or not fireing on time..

 

[MudderChuck]

Initial setup, for the 88 XJ. First the quick methode. Use a compession gauge, as soon as pressure starts to build, on the Num. 1 cylinder. Line up the timing marks (last turn to the right when facing front of engine) (question is, the farthest mark to the right or the second mark, have read both). Use an old distributor cap, cut a rather large window, showing the number one, cap inner electrode. Look and see if the rotor is, 0.020 past the electrode (rotor rotates clockwise). If not you will have to cut off the locking tabs at the base of the distributor, mark the distributor base and the block, so you can find your way back if necessary. I carefully trimmed one tab and cut off the other, makes, returning to initial position easier.
If your distributor cap is marked num 1, the orientation of the cam position sensor is probably right. If not I would, look in another distributor and make sure. Mark number 1 on the cap. Check the rotor for play, check the rotor for up and down play. There is gonna be some there, excessive play can be felt by comparing to another distributor.
Make sure of your firing order, to assume is not wise. Double and triple checking won´t hurt anything and might save a lot of time in the long run. If my tips don´t help, I´ll explain the long methode.

People, NICK, knock (pun) the Renix. It´s kind of an idiot computer, but sometimes simplier is better. Can buy a new one for under a hundred dollars. Can find one in a junk yard easially. They rarley mess up. You can have more than one failed sensor and they function (sometimes better :lol: ). I´ve unplugged my cam position sensor, most times the motor will run better.

 

[Jason01]

Well, I found one problem. The Crank position sensor had a tiny little chip on the end of it. Funny thing was that it was giving off a great signal, allmost 1 volt peak to trough on my scope. I checked it on a hunch. Now, with the first problem gone, it has developed a second problem. Im going to think that it is the timing on this one. At half to full throttle, or above about 2500 RPMs, It runs great and everything is hunkey-dorey. All most as soon as it shifts into 3rd gear, and you have to touch the throttle, it starts to sputter, and fire through the intake. Im not an idiot when it comes to engines(just some jeeps), but if the valves were leaking, wouldnt my symptoms be the opposite way around?

About the timing(I think that this is my problem), I had 1 jeep mechanic tell me that I'm a blazing idiot if I were to remove the tab on the dist so that I could adjust the timing. Of course, I had 3 others tell me to cut it off, and cut a hole in a dist cap(right around no. 1 cyl) and turn the dist. until the trailing edge of the rotor was .020 past(just like mudderchuck said). Would the timing cause this new problem I have discovered?

 

[Jason01]

Ooook, I checked the timing. It was a little off, BUT it still aint right! It runs great at very light throttle, but when it shifts, it pops through the intake. Another thing is that when it starts, the idle flares to about 2k RPM, and then dies. start it again, flares, and dies. On the 3rd try, it always starts, flares, and slowly comes down to about 800-1100 RPM's. I'm beginning to think vacuume leak? Any other ideas?

one other thing, when you drive it, it is runs great when it is floored, or running above 2800 RPM's. Driving this jeep in 1st and 2nd gear all the time at 30 mph just isint feasable. None of the pro mech's can help any. There is one who swears that the only way to get fire in the intake is if the valves are leaking. He also said that it would die when you floor it, because of the extreme cyl. pressure which would create more blow by the valves. Any Ideas? Please help me before the jeep is parted out on Ebay!

 

[XJNick]

Nick Knocks the Bendix/Renix system not because I hate it, but because it's just such a shame it doesn't store codes. I agree with you Mudderchuck, it IS super easy to fix, once you've found the problem. :mrgreen:

I personally though, really like the modern OBD-II systems, as they all adhear to the same industry standards and all work very similarily, regardless of vehicle manufacturer. Universal Compatibillity is awsome :mrgreen:

-Nick :!:

 

[MudderChuck]

When it idles up high, it´s most often a vacume leak (intake or the line to the vacume canister). The vacume leak supplies too much air, the 02 sensor idles up to compensate, there is a conflict between RPM´s, the throttle position sensor and what not. The computer idles it down, by a best guess process, so you don´t drive your auto transmission through a wall.
There is a recall on the 88, for cross fire (the symtoms you describe). Re indexing the distributor is the cure, sometimes. Fire back through the throttle body, could be a valve, which would show up, on a compression test, or as a wild needle on a vacume test, or a leak down test. The best test is a vacume test, you can see the needle vibrate rather radically (should read around 12-14 inches at idle). The leak down test, should be done with low pressure and the best methode is to block the throttle body open put your ear down there and listen for the air whistleing past the valve, if the valve is really bad you can feel the air on your check.
Popping back through the carb. is fairly common with the XJ, fairly high ignition voltage and a small distributor cap, a little corrosion, less than 5 ohms of resitance on the coil high voltage wire, wrong index on the distributor (isn´t really timing, it´s indexing the cam position sensor). You get a cylinder full of fuel vapor and a plug fires with the valve in the wrong position, it will sure enough cough out of the throttle body.
Block off the EGR valve vacume line with a plug and take it out of play, not really necessary, just make sure it´s fully seated (needle nose pliers, push the rod towards the intake, not to hard). Just to eliminate a variable.
Make an ohm test on all of your plug cables (less than 200 ohms, far less is normal), check your firing order. I´m not thinking your incompetant, but I´ve skipped steps and made simple mistakes (happens), just found a plug cable I forgot to fully seat, the hard way, with my fingure. Sure made the motor run bad, and popped through the distributor some.
Simple valve timing test, TDC 1. try and spin the intake and exhaust push rods, wiggle them a little, should be some play. If one is very tight or both, could be your cam is indexed wrong. Gets frustrating, stay with it, step by step, write down your tests and findings.

 

[Jason01]

Hmm, I think I found part of the problem. We cought the IAT sensor changing by about 8k ohms. It would go up to about 11k ohms, and then just drop to like 2.5k ohms. Weird, huh? If you hold the throttle steady at about 2k RPM's, the engine would speed up to above 3k RPM's, or maybe just spit and sputter and die. If, you run at full throttle, it runs great. Any Ideas?

Another thing, I am getting about 17 in. hg. vacuume at idle. The guage flinches a little, but im going to write that off as normal, as it has a torque cam in it, and the vacuume is changing by less than 1 in. hg. Just to be sure, I plugged the guage on my 2.8 V-6 blazer and saturn, and both did about the same. I'm not too worried about the vacuume guage anyways.

 

[bremery88]

Don't know if your still having problems, but it sounds like the computer bit the dust.
S
ounds similar (not in all repects to what my 88 did). A garage that has a snap on scanner should be able to find the fault codes, but this really will not help you if it is the computer.

If you can, get someone with a cherokee and swap the computers.

On my jeep, the computer you only run at full throttle or it would sputter and die. We put in a new distributor and it did not help.

I got a aftermarket computer for 300 bucks, but "borrow" another computer first.

 

[Jason01]

mine will idle, but it seems as if the timing is erratic. If you put a timing light on it, it will go all over the place. The IAT sensor helped, but it still backfires through the intake. I really think that it has something to do with the computer, or the inputs. I am wondering, would the ignition module cause the jeep to do this? How can I test it to see if it is working? Has anybody else had this problem?