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yoda604
02-09-2012, 02:06 PM
Hey everyone,

I know I don't post much, but I do peruse every now and then. But I'm wondering if someone can help me because I'm at my whits end. I've had some people tell me that there are some pretty technically savvy people here

I have a 1990 Cherokee, 4.0L that I rebuilt the entire top end. The head was completely redone at lordco machine shop, It has a new camshaft, timing chain and pulleys, lifters, push rods, rockers, valves, guides, seals, springs, all gaskets and seals, tps, motor mount, distributor, cap, rotor, plugs, wires and cat converter.

As of now the engine runs, and if you step on it it goes like a bat out of hell. If you want to drive at around 70kmh it's fine and as long as you're accelerating it's fine too. But if you need to drive the speed limit, or in a school zone, it coughs and backfires badly.

The 02 tests fine, the fuel at the fuel rail tests 34psi, the timing is dead on, all sensors read perfect. The only thing that I can find is drop in vacuum mid range rpm. I figure it's the culprit, but I can't figure why it's there or were it's coming from. The spark plugs used to be burning hot but after some driving have gotten that nice color.

It's eating my soul. If anyone has any input it would be greatly appreciated

bcmaxx
02-09-2012, 02:23 PM
check and double check your distributor, I had a similar issue after a full rebuilt, I checked , and double checked and it came down to it being a tooth out, it even looked lined up but it wasn't.

yoda604
02-09-2012, 02:27 PM
It has been quadruple checked, jumped forward a tooth and back a tooth. When it idles too, it purs like a kitten, well a manly kitten.

bcmaxx
02-09-2012, 02:30 PM
map sensor, plastic line going to map, or where it plugs into the throttle body? all the ground wires intact between the dipstick tube and the harness on the passenger side? one of these is for the iac

yoda604
02-09-2012, 02:44 PM
I actually replaced all the ground wires, cleaned all surfaces. I have fixed shit before put it together and had problems with grounds so I didn't want to fuck with that again.Checked the line going to the map sensor and checked the sensor itself. I hooked it to my buddies super computer/code reader thing-a-majig and took it for a drive. I kept an eye on the timing and while I was accelerating it was a steady line and when I tried to cruise and it would cough and spit the line would be jumping and shaking like a dog shitting razorblades.

could it be coil?

bcmaxx
02-09-2012, 03:11 PM
was the motor out? tried another crank position sensor? I broke mine when I took my engine out ( bellhousing). And all four of those little wires are intact that go between the dipstick tube mount bolt and wiring hrness at firewall? I had one broken, but not visible, caused me problems until I finally found it

yoda604
02-09-2012, 03:18 PM
was the motor out? tried another crank position sensor? I broke mine when I took my engine out ( bellhousing). And all four of those little wires are intact that go between the dipstick tube mount bolt and wiring hrness at firewall? I had one broken, but not visible, caused me problems until I finally found it

The crank shaft position sensor checks out perfect. If I have time tonight I will double check those grounds. I believe one of them is for the magnetic pick up on the distributor, so it could definitely cause problems. When I look I will let you know

yoda604
02-15-2012, 10:59 AM
Just to give some closure, bcmaxx you were right. It was the MAP sensor. Although testing it showed proper there was a corruption in the feedback to the computer. I unplugged the vacuum going to the sensor and it ran beautifully, so I replaced the sensor and voila, no backfire.

youngun
02-15-2012, 11:02 AM
Drive it off a bridge and get a toyota, thats all I have to add... :D

bcmaxx
02-15-2012, 11:19 AM
Just to give some closure, bcmaxx you were right. It was the MAP sensor. Although testing it showed proper there was a corruption in the feedback to the computer. I unplugged the vacuum going to the sensor and it ran beautifully, so I replaced the sensor and voila, no backfire.
Good to hear!! [cheers]

yoda604
02-15-2012, 04:46 PM
Drive it off a bridge and get a toyota, thats all I have to add... :D

Funny, my other ride is an sas'd, 1990 4runner, with 35's.

I swear, I'm gonna get a sticker on the jeep that says, "my other ride is Toyota" and on the toyota, "my other ride is a jeep". I don't get what's up with all the hoopla between jeep and toyota. The best vehicles you ever see on the trails are either jeeps and/or toyotas. I'm not bias, they both have their faults.

Hellboy
02-15-2012, 05:50 PM
Funny, my other ride is an sas'd, 1990 4runner, with 35's.

I swear, I'm gonna get a sticker on the jeep that says, "my other ride is Toyota" and on the toyota, "my other ride is a jeep". I don't get what's up with all the hoopla between jeep and toyota. The best vehicles you ever see on the trails are either jeeps and/or toyotas. I'm not bias, they both have their faults.

Well said