Engine miss after startup....

[ComancheKid45]

Alrighty guys, i got another 1 for ya to solve :smart:

 

After some slow wheeling or even a decent drive that gets the engine up to temprature, after the engine is shut down and sits for a few minutes when I goto restart the truck fires right up but usually acts like its missing on 1-2 cylinders for about 15-30 seconds than it clears up and runs perfectly normal. The truck is an 88 4.0 AW4.

 

Any ideas on any specific area to check out, All plug wires are in tact and in good shape, the plugs were replaced in Jan. and the dist/rotor are only a few months old and as mentioned this only happens on restarts after the engine has had a few minutes of heat soak than clears up to normal withen 15-30 seconds and will continues to run fine untill the next hot restart...

 

Thanks for any ideas as always! ;)

[JeffN]

Might be your temp sensor telling the pcm a signal that it is too hot and leaning it out until fluid gets moving or even the o2 sensor

[ComancheKid45]

The Temp sensor was replaced when i upgraded my gauges in March 2010, the o2 sensor was replaced back in Jan with the PCM. :dunno:

 

Thanks though.... ;)

[ComancheKid45]

Anything else I could try to check.....this issue really has never been resolved :dunno:

[87Warrior]

Sounds like injector heat soak to me. I know this is for newer rigs, but take a look at it:

 

TSB: 18-031-0

http://www.wjjeeps.com/tsb/tsb_wj_1803103.pdf

 

My TJ suffers from this and I have yet to install insulation around the injector #3. Others who have reported the issue and utilized the Mopar fix have had success.

 

Seems like the engine heat causes a vapor lock at injector #3 (though I suspect all injectors could be affected), thus not creating a smooth burn in the cylinder making it feel like a miss. Once fuel flows through, the vapor is gone and there is no more miss fire.

[Sir Sam]

I was gonna say it sounds like the heat soak issue on the 00+ 4Ls, though I don't know why that would affect yours.

[ComancheKid45]

I installed a set of Precision Injectors in summer 2008, maybe the newer design of those over the OEM could possibly cause that.......its definently heat soak related, becuase I can let the truck idle/run all day long in the heat with both fans running and it never skips a beat......its only after its been shut off for a few mins...

[ComancheKid45]

Man i hate bumping this back up, but the warmer weather is upon myself and this issue still continues to haunt me. I'm about out of options but figured id refresh this for anyone that might wanna chime in...

 

Within the past year the following have been replaced and or swapped with another known good peice, anything related to the truck running ill post up.

 

*New*

ECU (due to failure in 1/11)

Plugs/Wires/Cap/Rotor

Ignition coil (but not the ICU)

Fuel Pump/sender

Fuel Pressure regulator

O2 sensor

EGR Valve

Map sensor

Crank sensor

IAC valve

Alternator

Battery

Battery/Alt Cables

 

*Swapped*

Coolant temp sensor on side of block

IAT sensor

 

I drove a bit this evening and then let the truck sit, I came back a few mins later fired it up and popped the hood to watch if i could spot anything while it was throwing its fit. Didnt see anything but the #4 spark plug wire arc once, and it was definently getting fuel...I'm pretty much ruling out the missing being due to fuel delivery but more towards ignition, and all thats left on that side of things is the Distributor itself. I'm gonna swap that out this weekend with 1 from the parts Jeep and see if that makes a difference, i also noticed if i unplug the distributor from the harness while the truck is running i noticed no ill effects, but I'm not sure if thats pointing me towards anything or not.

 

Anyway, thats where i am now....if you have any ideas post em up, anything new to try I'm all for it. ;)

[cruiser54]

Probably not the distributor.

The injectors you have should be fine with the heat. Mine are, and I live in Arizona.

What happens if you turn the key to ON and let the fuel system prime before turning to START?

 

Have you ever checked your sensor ground circuit for high resistance?

[ComancheKid45]

Thats my starting routine.....I always give it 3-5 seconds before i crank the engine after the key is turned on. Where excatly is the ground i should be testing?

 

Thanks!

[cruiser54]

Under "important note" in the following write-up:

 

RENIX TPS ADJUSTMENT

 

Before attempting to adjust your TPS be sure the throttle body has been recently cleaned.

It's especially important that the edges of the throttle butterfly are free of any carbon build-up.

 

IMPORTANT NOTE: Set your meter to measure Ohms. 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.

 

If you see more than 1 ohm of resistance, and/or fluctuating resistance, some modifications to the sensor ground harness will be

necessary. The harness repair must be performed before proceeding.

I can provide an instruction sheet for that if needed.

 

MANUAL TRANSMISSION:

 

RENIX manual transmission equipped XJs have a three-wire TPS mounted on the throttle body.

This manual transmission vehicle TPS provides data input to the ECU. The manual transmission

TPS has three wires in the connector and they're clearly embossed with the letters A,B, and C.

 

Wire "A" is positive.

Wire "B" is ground.

 

Key ON, measure voltage from "A" positive to "B" ground by back-probing the connectors..

Note the voltage reading--this is your REFERENCE voltage.

 

Key ON, back-probe the connector at wires "B" and "C". Measure the voltage. This is your

OUTPUT voltage.

Your OUTPUT voltage needs to be seventeen percent of your REFERENCE voltage. For

example: 4.82 volts X .17=.82 volts. Adjust the TPS until you have achieved this percentage. If

you can't achieve the correct output voltage replace the TPS and start over.

 

AUTOMATIC TRANSMISSION:

 

RENIX automatic transmission equipped XJs have a TPS with two connectors. There is a flat three-

wire connector, same as the manual transmission vehicles have, and it is tested the same as the

manual transmission equipped vehicles--FOR ENGINE MANAGEMENT RELATED ISSUES.

 

However, the automatic TPS also has a square four-wire connector clearly embossed with the letters

A,B,C, and D. It only uses three wires and provides information to the Transmission Control Module.

 

Key ON, measure voltage between "A" positive and "D" ground. Note the voltage. This is your

REFERENCE voltage.

 

Back-probe the connector at wires "B" and "D". Measure the voltage. This is your OUTPUT

voltage. Your OUTPUT voltage needs to be eighty-three percent of your REFERENCE voltage.

For example 4.8 volts X .83=3.98 volts. Adjust the TPS until you have achieved this percentage.

If you can't, replace the TPS and start over.

 

So, if you have an automatic equipped XJ your TPS has two sides--one side feeds the ECU, and

the other side feeds the TCU. If you have TRANSMISSION issues check the four-wire

connector side of the TPS. If you have ENGINE issues check the three-wire connector side of

the TPS.

 

For those with a MANUAL TRANSMISSION--the TPS for the manual transmission XJs is

stupid expensive. You can substitute the automatic transmission TPS which is reasonably priced.

 

Revised 03-13-2012