Sputtering At Low Rpms

[Egm89]

Hello! I would like to ask the gurus of jeep mechanics in this forum, for help on sum issues that I have on my mj.

 

here's the list of symptoms:

Lost of power

Sputter at low rpms

Rough idle

Horrible gas milage

 

What I've changed so far:

Map sensor

TBI unit

Gas pump with the strainer

Gas filter

 

What I'm going to change:

Vaccum hose, (all of them)

Oxygen sensor

TPS

 

Thanks in advance, and it feels great to be part of this community!

[Oddmodman]

Well, honestly I don't have experience with this on Jeeps, but I fixed this issue on my '93 Chevy K1500 4.3 last month with a new set of plugs, wires, rotor, and cap. First I changed the plugs, and it got a tiny bit better. Then I changed the cap and rotor, and it got a tiny bit better. Still didn't quite fix the problem. Then I changed the wires and that fixed it all! So, just a thought. I'd check your plugs from damage, and if, like mine, they're black and rounded off, I'd atleast replace the plugs and wires. 

[Egm89]

Thanks for the input!

Well, as far as I know its a freshly rebuilt enine, so they have to be new, BTW its an 1986 2.5 stick mj.

[cruiser54]

Those vacuum hoses near the tB can cause all sorts of issues.

[Oddmodman]

Not to throw another negative wrench into the equation, but these old electronics systems are notorious for shorts and weak contacts, causing all kinds of havoc. Look into Cruiser54's (previous poster) http://comancheclub.com/topic/36382-cruisers-renix-tips/   for electrical cleaning instructions. Is it just a rebuilt block, or were all the vacuum hoses replaced at the same time? CPS is also a possible culprit.

[Oddmodman]

Oh, auto or manual?

[cruiser54]

Not to throw another negative wrench into the equation, but these old electronics systems are notorious for shorts and weak contacts, causing all kinds of havoc. Look into Cruiser54's (previous poster) http://comancheclub.com/topic/36382-cruisers-renix-tips/   for electrical cleaning instructions. Is it just a rebuilt block, or were all the vacuum hoses replaced at the same time? CPS is also a possible culprit.

The ground strap at the back of the head is even more critical on the 4 banger for some reason. Also, a new engine has fresh paint on it.....

 

 

Renix Ground Refreshing

The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, failed emission tests, and wasted money replacing components unnecessarily.

The components grounding at the dipstick tube stud are:

Distributor Sync Sensor, TCU main ground, TCU "Shift Point Logic", Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.

The factory was aware of the issues with this ground point and addressed it by suggesting the following:

Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.

Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.

While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.

Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.

First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.

Remove the other end of the cable from the rear of the head using a 3’4" socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.

A suggestion regarding the braided cable:

I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18" long with a 3/8" lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.

A further improvement to the grounding system can be made using a #4 cable, about 10" long with 3/8" terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.

For those of us with Comanches, it’s very important to remove the driver’s side taillamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal. Reattach securely.

If you want to upgrade your grounds and battery cables in general, contact Jon at

 

 

www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.

　

Revised 03-04-2013

[Egm89]

Thank you all for the input!

After I read all the feedback I whent to check al the ground terminal near the dipstick, and guess what my problem was?

One of the spark plugs cable was loose, plug the sucker back in, and badda bing badda bum! Some sort of wichcraft happen and the problem was gone! Feel kinda dumb btw :/.

I appreciate all ur feedback and feels great to be part of this awesome community!

[cruiser54]

Thank you all for the input!

After I read all the feedback I whent to check al the ground terminal near the dipstick, and guess what my problem was?

One of the spark plugs cable was loose, plug the sucker back in, and badda bing badda bum! Some sort of wichcraft happen and the problem was gone! Feel kinda dumb btw :/.

I appreciate all ur feedback and feels great to be part of this awesome community!

Good. Now do the ground refreshing anyway and save yourself some future headaches.

[Oddmodman]

Haha. Don't you hate when you're thinking all technologically, wondering what malfunctioning sensor or ground could possibly be screwing with you, when in reality it's just a tiny physical adjustment? LOL. Nice.

[cruiser54]

Haha. Don't you hate when you're thinking all technologically, wondering what malfunctioning sensor or ground could possibly be screwing with you, when in reality it's just a tiny physical adjustment? LOL. Nice.

Happens all the time. Back to basics, eh?

[jimoshel]

Haha. Don't you hate when you're thinking all technologically, wondering what malfunctioning sensor or ground could possibly be screwing with you, when in reality it's just a tiny physical adjustment? LOL. Nice.

That's what makes it fun, interesting?

[Oddmodman]

Along the same lines, I just bought a '90 cherokee sport for $250. It was cheap because at about 1800 or 2k rpms it'd start to bog down. What I ended up doing was taking the CPS and soaking it in kerosene for a night. Next day sprayed it off with electriclean, and reinstalled it. Took it for a test drive, and it exceeded 3k rpms without a hitch. Cool huh?

[cruiser54]

Along the same lines, I just bought a '90 cherokee sport for $250. It was cheap because at about 1800 or 2k rpms it'd start to bog down. What I ended up doing was taking the CPS and soaking it in kerosene for a night. Next day sprayed it off with electriclean, and reinstalled it. Took it for a test drive, and it exceeded 3k rpms without a hitch. Cool huh?

Bought yourself some time. Is it a stick shift by chance? Will it rev over 3000 now?

[Oddmodman]

Of course it's a stick lol. Personally, the only use I have for any automatic is as a boat anchor. Yes it revs good, and it'll rev past 3k under load as well. Sorry to hijack your thread Egm89, but I am curious what Cruiser's got to say :)

[cruiser54]

Of course it's a stick lol. Personally, the only use I have for any automatic is as a boat anchor. Yes it revs good, and it'll rev past 3k under load as well. Sorry to hijack your thread Egm89, but I am curious what Cruiser's got to say :)

The reason I mention a stick is that the CPS attracts the metal shavings from clutch material to it. That reduces their output signal. Autos don't have that issue. That's why cleaning it fixed your problem, for now.

 

Renix CPS Testing and Adjusting

 

Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.

You should get a reading of  .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs  (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!!  That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out the upper mounting hole to 3/8” from the stock 5/16”, or slot it so the CPS bracket rests on the bell housing when pushed down. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

 

Revised 01-26-2013

[cz777]

Not to throw another negative wrench into the equation, but these old electronics systems are notorious for shorts and weak contacts, causing all kinds of havoc. Look into Cruiser54's (previous poster) http://comancheclub.com/topic/36382-cruisers-renix-tips/   for electrical cleaning instructions. Is it just a rebuilt block, or were all the vacuum hoses replaced at the same time? CPS is also a possible culprit.

this is no joke ! i had a broken wire going to the TPS ...talk about drive you nuts !!

[cruiser54]

 

Not to throw another negative wrench into the equation, but these old electronics systems are notorious for shorts and weak contacts, causing all kinds of havoc. Look into Cruiser54's (previous poster) http://comancheclub.com/topic/36382-cruisers-renix-tips/   for electrical cleaning instructions. Is it just a rebuilt block, or were all the vacuum hoses replaced at the same time? CPS is also a possible culprit.

this is no joke ! i had a broken wire going to the TPS ...talk about drive you nuts !!

That was found how?

 

[AMCJeepMJ]

I have to chime in by saying I did the block-to-firewall ground refresh above for my 4 cylinder this spring, and it starts easier and runs nicer than before the refresh.

[cruiser54]

I have to chime in by saying I did the block-to-firewall ground refresh above for my 4 cylinder this spring, and it starts easier and runs nicer than before the refresh.

It seems to be more noticeable on the 2.5s for some reason.