Rough start when warm

[foreman1063]

Truck started some new behavior the last couple of months. Starts and runs fine when cold. Take a short trip, shut off and restart, sounds and feels like it runs on 5 cylinders. Rev it up a few times and it finally smooths out, usually. Today it happened a couple of times while running around doing errands but on the last stop I thought it was going to leave me stranded. Turned over fine but would not start, from time to time, while cranking, it sounded like it might start but didn't. Finally after about 5 minutes and 5-6 attempts to start it finally did. It cranked and cranked, then slowly, roughly began to run, after a few seconds of revving it smoothed out.

 

Work done the last 6 months: New plugs, dizzy & cap, wires, timing chain, injectors, fuel filter, knock sensor, adjusted TPS, bla bla bla.

 

Any ideas? :dunno:

[cruiser54]

Biggest players, in order are MAP sensor, CTS, and IAT.

 

They share the same ground circuit. Try this.

Only takes a minute.

 

Cruiser’s Renix Sensor Ground Test

 

Set your meter to measure Ohms. Be sure the key is in the OFF position. Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS . The letters are embossed on the connector itself.

Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it runs parallel to the valve cover and also near the MAP sensor mounted on the firewall. If you have an 87 or 88 with the C101 connector mounted on the firewall above the brake booster, wiggle it, too.

 

You want to see as close to 0 ohms of resistance as possible. And when wiggling the harnesses/connectors the resistance value should stay low. If there is a variance in the values when wiggling the wires, you have a poor crimp/connection in the wiring harness or a poor ground at the engine dipstick tube stud. On 87 and 88 models, you could have a poor connection at the C101 connector as well.

 

Revised 05/03/2012

[foreman1063]

Thanks Cruiser, I'll give it a try. I have an 88 but oddly enough it does Not have the dreaded C101 connector. :???:

[cruiser54]

Test your CPS cold and get the numbers posted up here. It would be nice to get a reading when it's warmed up and not starting but I'll bet the cold test might tell us something.

 

 

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 it’s mounting holes with the first drill bit that just won’t fit through the original holes. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

 

 

Revised 11-29-2011