3500-3800 RPM missfiring/bogging out?

So after i finally got my jeep up and going after sinking it into a 3 ft puddle, now at about 3500-3800 RPMs i have an issue. It acts almost like its a rev limiter/misfiring. It bogs down bad and sounds like it is back firing. It feel like it is jerking a little bit. Also when i go to take off every now and then it feels like it is going to stall out.

New fuel filter, Newer plugs, newer wires, swapped every sensor to get it running from a running xj along with computer and harness.

A few questions to help the techies out...

What year of Jeep is it? What engine, tranny, etc...

Did you get any water in the intake?

You mention you replaced a lot of parts, was the donor vehicle the exact same year? Did it have any issues at that RPM range that you know of?

HTH!

hmmm, if you have power on the bottom end, I'd say its either inadequate fuel supply or not a hot enough spark.

so maybe check the fuel pressure @ rail. Could be that the pump is going south or a fuel line leak, or injector issues...

and check out spark delivery...coil, battery voltage levels, etc...

just my first thoughts. (It's what I'd check anyways...)

87 4.0 AW4 231 around 200K Same year donor vehicle. I noticed that the CPS that i got from the donor vehicle had the bolt holes notched for easy install. Could that of caused the sensor to read wrong? Fuel pressure seems adequate. I dont have a gauge though. Battery is fully charged. I havent checked how the spark is though.

Wish I knew more about Renix...

There's a lot more folks who know a LOT more than I do, but I have a couple of thoughts.

I know this may be a dumb question, but have you tested your TPS with an analog multi-meter? (I think that's the tool, check out the 'Renix Files' on this site for some sweet Renix info.)

I don't know if it's relevant, but have you touched the anti-knock sensor? I don't know if it'd effect you Jeep in the manner you described, but it's a sensor I know I'd overlook.

Usually the Renix computers are pretty tough, was your old unit fried? You also mention replacing the harness, which harness was replaced, and why? Did it short out?

i havent checked the TPS with an analog multi meter yet. I swaped out the knock sensor too. Along with the o2 sensor. My old harness had a few wires that got fried along with the ECU. If i plug my old ECU in my fuel pump wont prime and i have no spark. Thats the main reason i swapped all my sensors. Trying to figure it out and then saw the harness and then found out it was the computer. Then it would idle and then act like it was only firing on 3 cylinders. So i tried to swap my spare CPS with another one and that stopped and idles fine. A little low but idles. And i drove it to take my daughters to see santa at the mall and found out if i am trying to get on it it stutters around 3500 to 3800 rpms

XJPhoenix said:

I know this may be a dumb question, but have you tested your TPS with an analog multi-meter? (I think that's the tool, check out the 'Renix Files' on this site for some sweet Renix info.)

Click to expand...

^ This.

I've had "bad miss" problems similar to what you describe caused by a "flat spot" in the TPS function. An analogue ohmmeter is the tool you want - you're less interested in an "absolute" reading than you are in the consistency of the change of the reading in response to wiper motion. Move it slowly back and forth to test. If the meter does not move, try a different set of terminals (I don't recall which two you want, but you only have three to choose from. That makes the most likely sets A/B, A/C, or B/C.)

XJPhoenix said:

I don't know if it's relevant, but have you touched the anti-knock sensor? I don't know if it'd effect you Jeep in the manner you described, but it's a sensor I know I'd overlook.

Click to expand...

A knock sensor won't cause a miss, unless the engine is already knocking. A simple test to rule it out? Put a timing light on, and note where the cylinder fires. Watch the timing while you tap the block with a hammer somewhere close to the knock sensor. If the timing changes significantly, the knock sensor is functioning properly. (I can never remember whether it's supposed to advance or retard...)

XJPhoenix said:

Usually the Renix computers are pretty tough, was your old unit fried? You also mention replacing the harness, which harness was replaced, and why? Did it short out?

Click to expand...

Confirm - what subharness did you replace? The only thing I can see happening to the RENIX computer would be that you flooded it - the only three failures I've heard of on several boards worldwide in the last ten years has been blown injector drivers - and I have two of them here in the garage (I want to check out Mr_W's repair for myself one of these days...)

AJTorris said:

i havent checked the TPS with an analog multi meter yet. I swaped out the knock sensor too. Along with the o2 sensor. My old harness had a few wires that got fried along with the ECU. If i plug my old ECU in my fuel pump wont prime and i have no spark. Thats the main reason i swapped all my sensors. Trying to figure it out and then saw the harness and then found out it was the computer. Then it would idle and then act like it was only firing on 3 cylinders. So i tried to swap my spare CPS with another one and that stopped and idles fine. A little low but idles. And i drove it to take my daughters to see santa at the mall and found out if i am trying to get on it it stutters around 3500 to 3800 rpms

Click to expand...

super wierd. sounds like you've done everything I'd do, in your situation.

My TPS problem was very specific at highway speeds. I would get to about 65 on the freeway, lift a bit off the throttle, and the car would stumble just once or twice and then regain its composure. I checked my TPS sensor with a multimeter and found two dead spots near the throttle position I was having issues with. I spent $30.00 on a new TPS, and it absotutely fixed the issue.

This was a pure "zero" signal type of stumble, though, like the whole computer system dissappeared into another dimension and then came back...haha. Yours sounds more like a gas tank on empty, going around a turn, and uncovering the fuel sender, "Chug, chug, wheeze, cough, varoommm....", type of thing. Thats why I immediately thought "fuel supply or spark control"

Best of luck, buddy. I'll reiterate, that you seem to have done your homework and have done everything pertinant...Its probably going to be a "forehead slapper" :doh: "right in front of you" fix..haha!

SanDiegoOverland said:

super wierd. sounds like you've done everything I'd do, in your situation.

My TPS problem was very specific at highway speeds. I would get to about 65 on the freeway, lift a bit off the throttle, and the car would stumble just once or twice and then regain its composure. I checked my TPS sensor with a multimeter and found two dead spots near the throttle position I was having issues with. I spent $30.00 on a new TPS, and it absotutely fixed the issue.

This was a pure "zero" signal type of stumble, though, like the whole computer system dissappeared into another dimension and then came back...haha. Yours sounds more like a gas tank on empty, going around a turn, and uncovering the fuel sender, "Chug, chug, wheeze, cough, varoommm....", type of thing. Thats why I immediately thought "fuel supply or spark control"

Best of luck, buddy. I'll reiterate, that you seem to have done your homework and have done everything pertinant...Its probably going to be a "forehead slapper" :doh: "right in front of you" fix..haha!

Click to expand...

Same problem I had. How'd I catch it? I changed my shifting pattern (five-speed) and noted it happened at the same RPM range in all gears. Presto - cut it down to somethign in front of the flywheel. Thinking about it eliminated probably 9/10 suspects, and left me with the only things that would have anything to do with throttle opening/engine speed.

The TPS was easier to test. I need to devise a test (and a response graph) for the MAP sensor one of these days...

I changed the harness from the computer to the engine. Both parts of it. Fuel injector side and coil side. When i plug my old ECU in i have no fuel pump prime and no spark

any luck or updates?

no same thing still. gonna swap TPS today and then check it.

ok well i started messing with the TPS and got it to idle at 700RPMs. It hasnt done that since before i bought it lol. Also with the jeep running i left the two bolts loose on it and was adjusting it a little and got it to do the thing its doing at a lower rpm about 2500 but not as bad as the 3800 rpm one and it would go up to 3800 and do the same thing.

Ok so i checked my TPS and the Ohms is
5 to 2.2 on a and c
3 to 6 on b anc c.

and that is from closed to open throttle. It swept smothly no jerking at all

anyone have any other ideas... now the jeep wont run at all. It will start but low rpms then die

I am having the same problem, did the tps fix it?

Mine was stuck in a creek up to the bottom of the grille, it was 4 in the morning and 5 degrees. After wading in the creek for a half hour I had to ditch it.. It has been running fine for a couple days now. It started doing the same thing yours is this morning. It is way to cold here for water in the gas tank..

ya no i drove over to the coast and back so i have filled up the tank with new gas. I also had my head off and cleaned my valves because something got into the combustion chamber and melted on the exhaust valve. We checked the valve for straightness and cleaned it all up. My father in law is a machinest so ya. And now it wont even start.

check the firewall ground. . .