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Discussion Starter · #1 ·
91 YJ base model w 95 Xj 4.0l engine. Automatic 999. Only engine mods are a cold air intake and a throttle body spacer.

Long story short. Replaced head. Replaced nearly every external engine part. Alternator, water pump, thermostat, pulleys, distributor, wires, plugs, all new. Replaced Idle Air motor, and Throttle Position Switch, Manifold Air Temp sensor, Water Temp sensor on thermostat housing.

Engine starts, idles fast. 1500rpm. Check engine light comes on when started. And keeps throwing codes 22 and 23. Which are water temp sensor and manifold air temp sensor. Both are new sensors. So now what? Something in the wiring harness? Gonna have to track the wires to the ECM and see if there is continuity. Could the ecm be bad?

Checking all the vacuum hoses and so far so good, but I will know after I finish if there is a problem. Thinking there may be a fuel pump failing as well. It works great until I get to 65MPH, then it backfires and stalls. Will restart after a few minutes, so thinking a thermal stoppage on the fuel pump. Pressure is good until then.

So I am getting codes even though I fixed the listed sensors, it is idling high though I replaced the IAC and TBP, and the engine stalls at 65.
 

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Try spraying carb cleaner around your throttle body and manifold especially in thd back... sounds like you have air getting in one of these places.. too much air and the computer will compensate with more fuel.. triy pulling off the spacer maybe its catching on something.. wiggle ecm wires while running maybe a lose ecm wire.. thats were I would start
 

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First thing to do is back up and uninstall the cold air intake and the throttle body spacer and see if the codes go away. If so install them one at a time and see if any one of them is the culprit.
Also you cant rely on codes to tell you what's wrong , you need to have someone with a scanner that can read real time data off the computer to do a proper diagnosis. Codes only tell you the general area where to look not what is actually wrong.
Defective aftermarket parts especially sensors can be problematic I always try to use OEM parts when I can.
 

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Discussion Starter · #5 ·
PROGRESS!!! Replaced most of the vacuum hose connectors. Found one suspect on the Fuel Return Diaphragm. Started it up. NO CHECK ENGINE LIGHT for the first time in 3 months. Took it for a low speed run around my neighborhood. Ran pretty good. Idle down to 1000. Tomorrow I will take it on a highway run and try getting it up to speed. If it does not crap out at 67, I will be doing the happy dance.
 

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Discussion Starter · #6 ·
Took it for a test run out on the highway. It worked very well for quite some time, maybe ten minutes, then at around 68mph, it stalled. I waited five, it restarted. Five minutes later, it stalled again. Waited five, restarted. I was able to limp home. BUT, no codes. Check engine light off.

At this point, I am thinking fuel pump may be failing. That would not send a code, right? Seems like a thermal thing. Fuel pump gets to a certain temp and the thermal protector kicks it off. It has a thermal switch on it, right?
 

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Discussion Starter · #7 ·
A jeep guy in my neighborhood suggested that the fuel vent line that runs to the charcoal canister might be blocked. So I disconnected it and blew it clear with a compressor. I left the vent line disconnected from the charcoal canister in case it is blocked. Took it for a test ride. Right at 67 mph it started backfiring. I backed off and it did not stall. I got back home without further incident. I drove around the block one more time and it stalled about a block from home. Unlike recent times it would not restart and I had the indignity of towing it home yet again.

Several hours later it would crank and not start. Could be the pump finally failed I realize this is a very basic question but what is the easiest way other than it not starting to tell if the pump failed? Pressure test, but what is the procedure?
 

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Discussion Starter · #9 ·
Update: Was going to drop the tank and change the fuel pump, but a friend had a pressure gauge which we connected to the fuel rail, and taped to the windshield so we could drive and monitor fuel pressure. We got the engine to fail, and guess what. Fuel pressure not the problem.

But it backfired before stalling. Now here was new info for me. According to a very good jeep guy in my neighborhood, backfiring like that is usually an ignition problem. The ignition cuts out, and without a spark, sends fuel into the exhaust, which either detonates when it hits the exhaust manifold, or when the ignition restarts and ignites the fuel. So, we checked all the ignition components. When we wiggled the coil wire, it caused the stall with a backfire or two. So first suspect was the coil. But we found the wire to be the problem. Fixed it and the engine ran fine for a while without error.

Then it threw an O sensor code. We started checking the wiring and discovered the problem. When I installed the engine, I ran the wires to the O sensor over the front by the power steering pump and they got too close to the exhaust manifold and got burned up and were shorting. I rerouted the wires and replaced the burned sections.

The engine ran for an hour without issue. Drove it home without issue. Will do a longer road test tomorrow, but for now, things are looking good. The only potential issue is the shorts may have fried the O sensor. Will replace for good measure.

Owning a jeep is like living in a beaver colony. One dam thing after another.
 

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Discussion Starter · #10 ·
OK Still Stumped!

No codes. Since the last post I replaced the coil, injectors, fuel lines, fuel filter, fuel pressure diaphragm, Oxy sensor, power steering pump (unrelated). In addition, in the last month I also replaced the distributor, rotor, cap, wires, plugs, TBP indicator, idle motor, MAT sensor. Fixed a couple of vacuum leaks, and a bunch of stuff I cannot remember. Oh yeah, water pump, radiator, hoses, and thermostat. Cooling system working flawlessly.

Tonight, I warmed the bastad up, when at 180˚, I took it around the block. A mile out it started popping, backfiring, it lost power and finally stalled. Would not restart. It has been doing some version of this for the past three months since the engine swap.

I am out of ideas. I will check the timing tomorrow. Crank position sensor. Not sure where else to look.

Checked the fuel pressure. It idled at 33 lb, and increased as we increased speed. Up to around 40 lbs. SO fuel pump is good. Had the gauge on when it stalled, and it maintained pressure throughout, so we eliminated fuel pump as problem.

Any help would be appreciated. It is not showing any codes. CEL not on.

Ready to drive it over a cliff, if it would only start.
 

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Discussion Starter · #12 ·
replace the Crankshaft Sensor and i think all your problems will go away...i've had the exact same problem twice. I have the same model, engine and transmission
I replaced the crankshaft positioning sensor with a 12 buck ebay product. Did not rectify. However I ordered the OEM CPS and it started right up. Ran for a while just great. After a half hour of driving, it threw an oxy sensor code. Put it on the reader and it indicated a constant voltage of 4.6. Didn't change with the RPMs. So I guess I should have learned my lesson. I bought the cheap O sensor. Now I will order the OEM.

Took it on a long drive. Climbing the very steep hill headed back from driving around town, about 25 miles into it, I got a few of the popping sounds through the throttle body and exhaust. Gotta be a spark or timing issue, but I am not sure what to check. It does not do it at low rpms. Only when the engine is under load. Going to switch the coil to distributor wire tomorrow. Perhaps it is intermittent and as it warms up it looses contact. New coil, cap rotor, distributor, wires and plugs. So not sure what else to check.

I suppose the load backfiring could be related to the O sensor. But perhaps it is reading high because there is too much oxygen in the exhaust... and that is causing it to run rich at the high end?
 
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