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Hello, first off let me begin by saying thank you to Moses for your site. It has helped me fix a lot of issues with my jeep.

I have a 1988 jeep yj with the renix 2.5 tbi setup. The problem is that the jeep starts and runs but consistently spit, sputters and stalls. I can be driving down the road doing 55 mph and then it is like somebody turns the key off and back on again. it does this at idle as well.

Things I have already done/replaced: new cap, rotor, plugs, wires, coil, ign. Module, ign. Switch, and the cps.

I have gone through and tested the coolant temp sensor (for the ecu), intake temp sensor, map sensor. Also pulled the wiring harness from the engine compartment and went through each individual wire with my multimeter to ensure that they were good. I did this under the dash as well and cleaned all grounds. The throttle body i rebuilt about 6k miles ago. New fuel filter and pump.

I am at a loss at this point and i hope for some guidance. The only thing i can think of is the ecm is going out but not positive.

Thank you for your help, and i look forward to your reply

Thomas

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88yj25...Thanks...This could still be a poor ground like we discuss with these models.  Paint, corrosion, oxidized contact points and such can be causes for faulty grounds or intermittent opens.  Check battery terminal connections closely and the engine-to-body grounds once more.  It sounds like you've been very thorough, though one common issue is the engine ground that attaches near the dipstick.

If this is not an ignition or ECU related cause, I would consider the in-tank sock filter at the bottom of the fuel tank.  You share that you've changed the fuel filter and pump.  Did you clean or replace the sock?  Were there signs of debris or sloughing in the fuel tank?  Has the filter been in the system for 6K miles?  One tank of bad fuel with high water content can swell paper matrix in the fuel filter and an create obstruction.  The fuel filter is an easy fix.

To isolate fuel system from ignition issues, consider installing a gauge and "T" fitting at the TBI test port.  First check for consistent fuel pressure within range.  Then check for fuel volume flow with a bleed-off hose from the "T".  With a valve on the hose and a safe fuel-resistant catch can, start the engine with the valve closed to maintain proper pressure; then open the valve and observe the volume of fuel coming from the hose.  It must be ample with a steady stream, consistent enough to keep the engine running under any load or speed.

I'm betting on a fuel pressure or fuel flow volume issue.  Yes, the ECU could have an intermittent open, though this is a longer shot; ECUs do fail and develop solder joint fractures that cause intermittent troubles...The unpredictable nature of the miss makes me consider a fuel issue first.  Also, the O2 sensor could be an issue if old or exposed to a rich fuel mixture.  A clogged catalytic converter is always a possibility though usually accompanied by loss of performance at all times.

For a quick check of spark, hook up your timing light at each plug lead, one at a time.  At each plug, look for a steady, solid flash of the timing light with the engine idling.  If you can get the engine to misfire during this test, it will be easy to see whether the miss is spark related.  

Places to start...Keep us posted!

Moses

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Ty sir,

When I replaced thefuel pump I did the sock as well since it was the only way to keep the warranty with the pump. I was thinking along the same lines though and am going to go ahead and drop the tank in the morning. ( for anyone that is having to do this may I suggest that if you or a friend have a motorcycle jack it helps allot with fuel tank removal!) Also the cat is gone lol previous owner tossed it and it has a brand new muffler due to a broke pinion shaft that caused the driveshaft the remove the old one for me lol. It seems like I have good spark so I am going to do a fuel flow test and a pressure test on my next day off. Thank you for your reply and I will let you know what I find in the upcoming days.

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You're concerned about both pressure and volume of flow...Of course the ECU could be defective in the end, but the faulting symptoms raise doubts.  I like your strategy of ruling out a fuel issue first.  You'll at least know for sure whether fuel pressure and flow volume are okay.  

Also, check out the EGR valve function and be sure the plunger is seating.  O2 sensor malfunctions can also be trouble spots.  A rare but additional possibility is the wiring from the CPS if subjected to heat.  I installed an aftermarket header on our 4.0L XJ Cherokee, and the CPS wiring got too close to the tubes.  There was a clear misfire and stalling condition that developed soon thereafter.  (A non-destructive insulation resistance test with a Fluke 1587 meter or equivalent would have revealed a short to ground.)  At least worth a peek...

Moses

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Ok fuel flow looks to be good and pressure seems stable at around 14 psi. That is until it stalls or cuts out. Then it drops for a second and catches right back up. Tried a inline fuel pump yesterday from my truck (higher pressure but I dropped high pressure to 20 psi through a fpr  I had laying around before I got it to the throttle body.) I ran this pump from a separate battery source and still had the same issues so I am fairly sure it fills out the fuel system. I am going back in today and going through the grounds again. Also checking what you have suggest above.

Thank you for your help thus far.

Thomas

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Ok finally back, had that hurricane roll through and finally getting back to the jeep.

I checked all above and all of that looked ok except for a small vacume leak but got that fixed. Still having the same issue. I did a little playing around today with a aftermarket voltage gauge just hooking it up and driving and found something interesting. At idle I have 14 volt,s but on the road at 2k rpm I am at 17 volts. thinking that the computer is getting 17 volts, and it may be throwing off the voltages going to the sensors, and the input voltage back to ecu. The only thing that throws me off about this is that at idle it still stalls as well even though I'm only at 14 volts at that time. Also going to have the battery tested tomorrow.

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Also one more thing I found out that the problem originator (aka previous owner) had a rebuilt engine installed into the jeep and it looks to be a newer engine 91-95. The reason i say this is that the crank shaft pulley sticks out a boos 1/4 to a half inch farther than the rest of the pulleys. If this is the case and it has the distributor from a 88 yj in a block with a cam shaft from a 91-95 engine would there need to be a change in the timing procedures for this engine or are the camshafts the same?

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88yj25...If this is a true reading of system or actual battery voltage with the engine running at 2000 rpm, it is very high. (Symptomatically, the battery water drops steadily and bulbs would be burning out prematurely.)  This kind of voltage would be damaging.  

Even in cold weather at start-up, alternator output at the battery should not exceed 15.3V for your Jeep.  At normal ambient temperatures (50-100 F), the alternator should put out 13.9V-14.9V, even less as ambient (including under hood) temperature increases.

Make sure this is the actual system voltage.  A reading taken at the starter motor end of the battery positive cable would be useful.  This is safely away from the battery for testing but provides an accurate battery state reading with the charge circuit operating.*  

*Warning:  With the voltage as high as you're suggesting, the battery voltage could be producing and venting dangerous gases that could explode if a spark were present.  Do not attach test probes or clips anywhere near the battery.

Try this approach and see if the voltage is more in line.  If not, there is a charge circuit/voltage regulator issue.  If necessary, there are a number of in-chassis tests you can run on the alternator plus a number of off-the-engine bench tests.

The pulley issue sounds like the difference between the V-belt pulley versus a serpentine belt pulley crankshaft.  Which type drive belt(s) do you have on the engine?  We can go from there...The camshaft should not be at issue, although spark base timing is important and must be set properly for your TBI 2.5L.  There is a camshaft change in 1996 on the 2.5L/150 engine.  We can look at that further if necessary.

Moses  

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Yes i plan on doing more testing on the voltages today. Current testing is on the wire going to the fuel pump and also the terminal on the start relay at the firewall where all the fuse links converge. The engine is equipped with the v belt drive system, the serp. belt crank is the reason I was asking since i knew it sticks out about 1/2 more than the v belt crank. I will update in morning with what I find. Thank you for your help thus far.

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88jy25...If the gauge/tester is accurate, these are each good test points, and the voltage is very high.  Verify that this voltage meter works properly.

I'm not clear why the damper sticks out.  The long crankshaft snout is actually the V-belt engines.  The serpentine belt engines, of which all later engines fall into that class, have a shorter crankshaft snout.  When a stroker 4.2L V-belt crankshaft (longer snout) is installed in a serpentine belt 4.0L engine, there is a special washer available from HESCO Jeep to compensate for the stick-out space:  http://www.hesco.us/products/7735/cylinder-heads/45565/crankshaft-damper-spacer-hesbs#.V-VnGfkrKUk.

I'm curious why the pulley sticks out.  This could be misalignment of the upper driven accessories, an odd mix of engine-driven accessories or a mismatched water pump type and pulleys.  Worth confirming what's going on.  Belt misalignment will lead to premature accessories and water pump wear.

Moses

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