Brett777 Posted July 1, 2010 Share Posted July 1, 2010 Hello guys! I have a 1991 Jeep Comanche 4 wheel drive with the 4.0 HO MPI engine.............. This is a new occurrence within the last 3 days. When I press down the accelerator the engine takes a very long time to accelerate. It is as if I am hauling a very heavily loaded trailer. I have checked everything I can from dragging brakes to see if a boat anchor for an aircraft carrier has been hooked to the rear bumper, replaced the MAP sensor, TPS, idle mixture controller, O2 sensor, checked the vacuum at idle and when driving, fuel pressure at the rail and pump at idle and driving, etc, and still I have this problem. I cannot climb a hill even at a 1% grade without losing speed. It is as if the the effective throttle range is only the first 25% of throttle and after that it has no effect. I also notice that if I press the throttle half way or even fully depressed the engine coolant temp rises about 25-30 degrees. Spark plug check showed that the crease in coolant temps. I have checked everything I could including valve lift at the rockers to see if a cam lobe has been worn out and everything seems to be within spec. Any help would be greatly appreciated. Link to comment Share on other sites More sharing options...
Pete M Posted July 1, 2010 Share Posted July 1, 2010 clogged cat? Link to comment Share on other sites More sharing options...
Brett777 Posted July 1, 2010 Author Share Posted July 1, 2010 Unfortunately no. I took it off an installed a "test" pipe and the same thing is still happening. Link to comment Share on other sites More sharing options...
Pete M Posted July 1, 2010 Share Posted July 1, 2010 fuel filter? bad gas? Link to comment Share on other sites More sharing options...
Brett777 Posted July 1, 2010 Author Share Posted July 1, 2010 All of that has been checked and or replaced. Let's put it this way.......I am eyeballing one of my 406 small block Chevy engines to put in it like in my CJ5!!!!! I don't want to but this problem Is making no sense to me Link to comment Share on other sites More sharing options...
Automan2164 Posted July 1, 2010 Share Posted July 1, 2010 Does the TPS sensor have voltage the whole range through? Rob L. Link to comment Share on other sites More sharing options...
mythreesons Posted July 2, 2010 Share Posted July 2, 2010 Hello guys! I have a 1991 Jeep Comanche 4 wheel drive with the 4.0 HO MPI engine.............. .........It is as if the the effective throttle range is only the first 25% of throttle and after that it has no effect......... Warning!!!!!! Silly question follows!!!!!! You didn't say if your running the automatic or manual, but I am guessing auto. Does the throttle work appropriately in park or neutral? More silly questions!!! Did you check the TCU (if auto)? Check the rear dif? Did you try talking nice to it? (Sometimes, that works. :D ) Scott Link to comment Share on other sites More sharing options...
Brett777 Posted July 2, 2010 Author Share Posted July 2, 2010 This truck has a 5 speed manual transmission and the TPI voltage starts at .512 volts and tops out at 4.776 volts wide open. Link to comment Share on other sites More sharing options...
Pete M Posted July 2, 2010 Share Posted July 2, 2010 how many miles? original timing chain? Link to comment Share on other sites More sharing options...
Brett777 Posted July 2, 2010 Author Share Posted July 2, 2010 117841 original but only 17564 on the engine which is 4 years old. Link to comment Share on other sites More sharing options...
Pete M Posted July 2, 2010 Share Posted July 2, 2010 have you tested the fuel flow rate? pressure is one thing, but if there's not enough flow the engine will run lean. Link to comment Share on other sites More sharing options...
Pete M Posted July 2, 2010 Share Posted July 2, 2010 what about testing the sensor outputs at the computer? maybe something is wrong with the wire harness that's causing the computer do screw up. Link to comment Share on other sites More sharing options...
Brett777 Posted July 2, 2010 Author Share Posted July 2, 2010 This is where I started going from head scratching to going bald! Flow rates were checked from the pump tot the rail to each injector, just like I would with an aircraft FI system. Voltage levels at the computer were all good and I even calculated parameters for voltage drop based on wire size and length......................results= nothing wrong here either. Who wants to join me in going bald now? Link to comment Share on other sites More sharing options...
Pete M Posted July 2, 2010 Share Posted July 2, 2010 checked the timing? Link to comment Share on other sites More sharing options...
HOrnbrod Posted July 2, 2010 Share Posted July 2, 2010 Yes, you can do this with a timing light since the timing marks are still on the harmonic balancer pulley. UB looking for a nice steady mark with the light. Checked the distributor for proper indexing? Did this lack of throttle response come on suddenly, or was it a gradual thing? According to your posts, you've done all the right things and there's not much left. I didn't see anything regarding monitoring your A/F ratio throughout the RPM range either. Perhaps bringing it to a local shop with a dyno might be in order? Got a buddy who will loan you a compatible ECU you can plug in to eliminate that? Just throwing things out here.......... Link to comment Share on other sites More sharing options...
jeepdoggydogB Posted July 2, 2010 Share Posted July 2, 2010 Fuel pump :dunno: I had something very similar happen to my 91 four years ago, fuel pressure checked out when tested at idle and acceleration (I even purchased the DRB II scanner to verify numbers), the minute I took her on the street or freeway she seemed starved for fuel. I replaced everything as you did and nothing helped, as a last resort I replaced the fuel pump, and she ran like normal. Link to comment Share on other sites More sharing options...
Brett777 Posted July 2, 2010 Author Share Posted July 2, 2010 The fuel pump flow and pressures have been checked from idle to running at 55 mph and so has the timing as I replaced all the plugs, harness, cap, rotor, and coil. The ignition parts were replaced to to fix this problem but simply because I had these parts in my shop and figure what the hell.............I have darn near replaced everything else! The cockpit of my truck now looks like a rolling laboratory because almost everything has a probe or gauge attached to it that leads back to my test computer that I use when test flying aircraft. This has now become personal! I just drilled some holes in the exhaust system and installed some homemade piccolo tubes attached to pressure transducers as well as installing some aircraft EGT probes at the exhaust ports to see the pressure changes and the EGT changes when she runs out of steam. I also installed some piccolo tubes fore and aft the air filter to get some idea of what happens when she starts losing power on the intake side beyond 25% throttle. Sounds like a rolling dyno but like I said.......it is personal now! Actually, it only took me about an hour to install all these probes and I am very curious about what the data will show. Link to comment Share on other sites More sharing options...
jeepcoma Posted July 2, 2010 Share Posted July 2, 2010 Did you check the most basic of basics... the air filter? Sounds very similar to what my Bronco used to do, after a few hours of driving in the rain or snow. Turned out it was sucking water up through the grill and saturating the air filter element, causing it to run extremely rich. It got to the point where I'd be in 2nd gear WOT trying to get up a 2% grade on the highway, until I finally figured it out. Link to comment Share on other sites More sharing options...
Pete M Posted July 2, 2010 Share Posted July 2, 2010 Keep all those records! :bowdown: It'd be awesome to have all those numbers (once it's running right of course) so other members can reference them in the future. Link to comment Share on other sites More sharing options...
JeepinDev89 Posted July 2, 2010 Share Posted July 2, 2010 with the whole WOT thing not accelerating right. my 91 is doing the same thing. i thought maybe it the torque converter was loose but being that yours is a five speed i suppose that can't be it. but i'm going to keep an eye on this topic and maybe solve my problem as well. devin. Link to comment Share on other sites More sharing options...
Brett777 Posted July 3, 2010 Author Share Posted July 3, 2010 Well........the numbers are in!!!!!! Cause for the problem? Exhaustious pipeous interruptus.......or in layman terms.....the damn exhaust was delaminating approx 3 inches prior to the flange where the cat bolts too!!!!!!! After about 4 hours of labor and a brand spankin new custom exhaust system, she pulls like never before! The pipe clogged like a artery just prior to a massive heart attack. Additionally, there were bigger flaps of metal acting like valves further restricting the flow under acceleration. Boys.........its Miller time! Link to comment Share on other sites More sharing options...
Pete M Posted July 3, 2010 Share Posted July 3, 2010 :banana: :banana: :cheers: :bowdown: I knew it sounded like a clogged cat. But there's no way I coulda guessed a peeling manifold. :hmm: Link to comment Share on other sites More sharing options...
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