Sluggish Acceleration (RENIX)

I am getting frustrated with my 87 XJ. I am having a problem with sluggish acceleration and poor gas mileage. It passed the tough CA smog test for gross pollutors with flying colors, but it still has a loss of power. Prior to the smog test I performed a tune up which included:

-spark plugs and wires
-cap and rotor
-oil and filter
-air filter
-checked entire vacuum system (found several cracks and holes)
-fuel filter
-TPS (solved high and erratic idle problem)
-cleaned grounds (found the one from the block to the firewall hanging loose)
I plan on adding another one tomorrow on my day off.
-Dynomax cat-back exhaust (looked at cat and the medium was in good condition, the previous muffler was rotten)

This Jeep has been in my family since new, and I noticed the problem when I got it from my sister prior to the lift and tires. I assumed that it was in need of a tune up. So after a year and a half of building the Jeep in my garage, I took it out and was disapointed that the sluggish acceleration was still there. My sister had the fuel injectors replaced, 02 sensor, cat, and the head repaired (crack) a couple of years ago. She stated that she had noticed a power loss before she gave me the Jeep.

I have read the Renix Files by ECOMIKE.

I plan on checking the EGR valve, MAP sensor, and CPS tomorrow.

Please help, I have spent a lot of time and energy on this Jeep and I am dying to go play.

How built is it. If you have built it up may be time to regear?

My 88 will run neck and neck with my HO. The TPS seemed to be a big part of poor performance for me in the beginning (along with the O2 sensor). I did some fine tuning and worked some kinks out of the TPS ground circuits (bad spices and dirty connectors). Remember that the TPS is two parts (two halves) one for the motor and one for the tranny (separate grounds that both end up at the dipstick holder with more than a few splices hidden in the harness). Adjusting the TPS to the tranny side may not be addressing all of the problems, it's worth a test to see what the ECU side of the TPS is doing and how the grounds are.
My compression is acceptable but not good and it still runs strong.
You may want to test the sync sensor, a simple test. They rarely cause any problems, but can theoretically cause some fuel issues.
Fuel pressure/regulator?
The RPM range of the problem and the motor temperature are also indicators of where the problem may be. Idle, mid range or top end?
A new O2 sensor is most always a good place to start. They generally act up first at or near peak torque (around 2200 RPM) and affect top RPM's (above 3500), which could mess with your acceleration. They can starve your motor in certain RPM bands.
Has the distributor ever been out of the motor? Getting it a tooth off may not mess with your idle, but may mess with your timing advance.

I was hating my sluggish renix for a while, but found out the PO had the TPS on without making sure the throttle lever pushed down the TPS lever. I felt stupid that I never caught the problem, but as soon as I fixed the TPS throttle lever positioning, she races. Plus all the tuning I've done before and since fixing the TPS, I'm loving it. Also, check the Transmission "kickdown" cable. It can be adjusted to slightly modify the shifting profile.

Three things to check:

1. Throttle Position Sensor
2. Throttle Position Sensor
3. Throttle Position Sensor.

Did I mention the Throttle Position Sensor? Ask me how I know.!!!1

I have 35" tires with 5.13 gears, before the build it had 4.10 with 30" tires. I installed a new TPS as previously mentioned, adjusted to 0.816. I forgot to mention that its has a manual tranny, not auto. Idle is great, but acceleration gets continually worse as I shift through the gears. I have to wind out 4th to be able to drive in 5th, and I have to floor it in 5th to hold 65 MPH. All of these problems were present before the build up and tune up.

I am off to the garage to add the extra grounds and check the wiring from the TPS to the ECU, and continue to check the many sensors.

Have you run any injector cleaner through the system? You might want to try Acetone. I think about 4-5 ounces of Acetone for the next couple of tankfuls would do your buggy a world of good. Some folks on here swear by Seafoam, but I've never tried it.

I am going to run Seafoam on the next couple of tanks, and see where that gets me. Thanks

not sure how the gas is out there in CA , but here in PA just about every station adds that "ethenol" to it. which realy messes with the renix.
almost everytime we take our foor off the gas pedal she stalls. so after we move in 2 weeks im gonna be replacing the in-line fuel filter & running some injecter cleaner through her. im just hoping that that works for us.

corn grows for us to eat, nut burn in our vehicles!!!! (stupid polotitions)
no wonder the price of food has sky rocketed!!!

cracked-butt said:

not sure how the gas is out there in CA , but here in PA just about every station adds that "ethenol" to it. which realy messes with the renix.
almost everytime we take our foor off the gas pedal she stalls. so after we move in 2 weeks im gonna be replacing the in-line fuel filter & running some injecter cleaner through her. im just hoping that that works for us.

corn grows for us to eat, nut burn in our vehicles!!!! (stupid polotitions)
no wonder the price of food has sky rocketed!!!

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I have no problem with the ethanol blends in my Renix.
Check the EGR, EGR solenoid, fuel pressure regulator ( is is vacuum operated and spring loaded!!!!!!) and check the TPS for a noisy operation.

My money is on the EGR system or fuel pressure regulator being bad on yours.

I guess the best description is that it stumbles on acceleration (including each time that I shift up), has an overall lack of power, and low fuel mileage. All of these problems were present before the build up and I thought that it was just in dire need of a tune up. When I found the rotten muffler I thought that might be the culprite. I plugged the vacuum from the EGR selonoid and still no change. The TPS is brand new and adjusted to 0.816.

What are the symptoms of a Crank Sensor or a Cam Sensor thats going out? I plan on checking the O2 sensor on my next day off, and the distributor to make sure that it is in the correct positionand not worn out. I also plan on running a compression check if I can get my testor back from my friend by then. I haven't checked the fuel pressure yet.

Does anyone know what the stock timing should be on a 1987 4.0? It feels almost like the timing is not advancing correctly. The stumble appears in the same RPM range at each shift and then smooths out.

I appreciate all the help, I am getting frustrated.

The timing should be self-regulating, unless the distributor has been out and put back in wrong position. How about disconnecting the knock sensor? If it is bad it could cause the ignition to be permanently retarded... Just a thought..

since you have a manual trans you don't have to worry about adjusting the tcm side of the tps sensor. Before doing anything I would suggest inspecting and cleaning the ECM & TCM ground connections located on the oil dipstick tube mounting area. They always get covered with dirt grime (oil?). The wire ground strap that goes from the bulkhead (firewall) to the cylinder head is just the dash panel to engine ground (as stated in all-data online). Then you must test your tps signal wire for any opens through it's full range of motion. It won't read close to 5V when fully open due to limits of the throttle body. You should check your timing with a timing light just to see if it advances. I painted a dash with white-out for better visibility on the harmonic balancer.
You should also check fuel pressure with the engine running should be around 30psi the. Disconnect the vacuum line @ the pressure regulator then it should rise to 40 psi. Check the bleed down of the pump check valve by shutting the engine of with the gauge connected. It should not drop & hold for 30 min (just a guideline not exact). Fuel pumps get weak & they can't either deliver the proper pressure or fuel volume required under high engine loads. A weak ignition coil could also be a cause. Symptoms of a bad cps are crank no start or engine stalling and no restart until the engine cools. A bad sync sensor will cause abnormally long cranking times. I recommend googling jeep reference J007 and viewing the PDF file & performing the service procedure as I did in my 89 so far no more starting problems (fingers crossed). Hope the info is useful.

I have the 2.5L in my '95, which also uses the Chrysler PCM, so this may or may NOT apply.

My sluggish acceleration and intermittent drop in power, feeling like a miss, improved with replacement of the sensors mentioned above, the TPS and temp sensor as well as the IAT sensor.

But, it was still there, even though it got better. Then I tried 89 Octane as opposed to the usual 87 Octane, and within a few minutes the power drop problems were gone.

I'm guessing, the power problems and sluggishness came from knock sensor sensing the beginning of detonation and the PCM retarding the ignition timing. Probably built up carbon over the years in the motor. The previous owner didn't take that great of care of the vehicle, when I pulled the manifolds, the valves were coated in carbon. I've done the SeaFoam long ago, it didn't solve the problem, only trying a higher octane completely solved the problem.

I've been ran a tank of Redline Fuel System cleaner and on my 3rd tank of lucas cleaner, and soon I'll try the Mopar Combustion Chamber Cleaner, which seems to do a little better job than SeaFoam at cleaner out the carbon. After that I may try 87 again, but it very well may be the motor needs a higher octane at higher miles, either the carbon build up or the ethanol thing mentioned earlier.

I appreciate all of the help. I hope I can get this cleared up soon, it's getting frustrating.

I just replaced the Crank Position Sensor (CPS) and did the Mopar Combustion Chamber Cleaner (CCC) last night. Whoa, its like night and day, no more hesitation, no more power drop, heck I can even accelerate in too high of gear (I have a Manual) and the motor will NOT totally bog down like it used too. Smoother idle also.

I'm sure it was replacing the CPS, Mopar CCC will NOT clean out all the carbon, nor will cleaning some carbon out produce such a drastic change in driveability. I've never had Mopar CCC do that drastic a change in other cars I've used it either.

The Haynes manual says there is a test for the CPS to tell if its good or bad. IIRC, for earlier models, pre-1992, your supposed to put an ohm meter on tabs A and B of the connector to the CPS. It should show no resistance (open). Yea, I know, open is infinite resistance and closed is no resistance, so which one is it? that Haynes for you. :roll:

Later models you do the same test but for tabs B and C. Since I already had a new one, I didn't bother testing the old one. The old one was also totally coated in sludge and grime, that may have had something to do with it also.

Next tank of gas will be 87 Octane, see if I can back to using recommended octane and still have the motor run like it should. If I've solved the problem, the extra octane is probably no longer necessary.

Possibly the cat is plugged.

The cat looked great when I had the exhaust apart to put the cat-back on. A friend of mine is supposed to be bringing over a fuel pressure gauge tomorrow to check the fuel pump. I think it may be going, or at least the part that holds pressure after you turn it of is gone. You can turn the key to the on position, and let the pump prime. Then turn it off, an immediately repeat and it still has to prime. I did this several times, and the amount of time it took to prime did not shorten. I noticed this when I was fixing the problem and eventually replacing the TPS. It doesn't matter if the engine has been running or not. If you tun it off then immediately turn it back to the on position, the pump still has to prime for a second or two. Not sure if this means that the pump is actually going.

Did I mention that I'm getting frustrated?

Cornfed-87XJ said:

The cat looked great when I had the exhaust apart to put the cat-back on. A friend of mine is supposed to be bringing over a fuel pressure gauge tomorrow to check the fuel pump. I think it may be going, or at least the part that holds pressure after you turn it of is gone. You can turn the key to the on position, and let the pump prime. Then turn it off, an immediately repeat and it still has to prime. I did this several times, and the amount of time it took to prime did not shorten. I noticed this when I was fixing the problem and eventually replacing the TPS. It doesn't matter if the engine has been running or not. If you tun it off then immediately turn it back to the on position, the pump still has to prime for a second or two. Not sure if this means that the pump is actually going.

Did I mention that I'm getting frustrated?

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That is normal, FP runs on an ECU virtual timer.

If you suspect the O2 sensor, just disconnect the electrical connector to see if the problem goes away, if it does, then the O2 sensor or wiring is causing the problem.

EGR valve could be stuck open!

Ecomike said:

EGR valve could be stuck open!

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x2

I know deleting my egr made a major positive difference!!! I was having the most trouble during cold starts, and that was most definitely the egr valve! the rest of my sluggishness problem was the TPS as mentioned earlier... between those two, you should be able to make alot of improvement in your performance problems. I was prepaired to start systematically replacing all the sensors, but it never got that far, thank goodness. I would start with TPS, then EGR, then CPS, down stream O2, upstream O2... and anywhere in between, you can disconnect the ECU, and clean the connections and reconnect it. The best place I know of that breaks down renix Diagnostics is lunghd.com in the on site, and off site tech areas of the website.