please help zj runs rough in the morning - JeepForum.com
 
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post #1 of 9 Old 08-19-2017, 08:01 AM Thread Starter
jdhint86
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please help zj runs rough in the morning

my 98 zj first thing in the morning the rpm's bounce for a little bit in the morning. I replaced the plugs,wires, distributor, tps, fuel pump, and both oxygen sensors. And it still does it. The fuel rail is empty in the morning but I thought the fuel pump would fix that. I'm not sure what else to try. The wierd thing is that it can sit at work for 8-9 hours and it will run fine when you start it up and go it's only in the morning that it does this. So any help would be great.

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post #2 of 9 Old 08-19-2017, 08:47 AM
Aifer
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Weak battery (or battery drain) can cause the step-motor to work crazy.

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post #3 of 9 Old 08-19-2017, 12:46 PM
Uniblurb
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What brand of fuel pump did you install? If you went with a cheap one or brands we've seen problems with (Airtex, AC Delco, etc.) that very well could be your problem.

The only brand of fuel pump module recommended is Mopar (repackaged Bosch) or the Bosch 67649 for the 97-98 which is OE. Here's a photo of it and it's $169 plus shipping on Rockauto.

http://www.rockauto.com/en/moreinfo....315963&jsn=388

While you should/may have zero pressure at the fuel rail on first start-up the fuel line should still be full of fuel and not empty. If not the check valve on the pump outlet is bad.

Also the fuel regulator which is in the 'can' w/filter on top of the pump module is supposed to be a secondary check valve system. On shut-down the diaphragm in the regulator is supposed to close helping keep fuel in the line. Lots and lots of problems with the Asian made fuel regulator/filters on the cheaper modules.

You may want to see if turning the ignition on w/o starting a few times to prime the pump makes it run smoother on first or morning startup. Good luck.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #4 of 9 Old 08-19-2017, 06:55 PM
mikem350
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OK, first, there are no CEL codes?

What fixes my 4.0s with a similar problem is cleaning the throttle body and IAC. Seems like cars like mine with hi mileage need cleaning every year or so....

Proud owner of two ZJs
Red 94 mint!
Silver 98 just hit 300k miles....runs like new!
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post #5 of 9 Old 08-20-2017, 08:23 AM Thread Starter
jdhint86
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the pump I out on it was a ac Delco from napa I 'll try to clean the throttle body better and where and what does the iac do
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post #6 of 9 Old 08-20-2017, 10:46 AM
Uniblurb
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I know NAPA normally sells high quality parts. But unfortunately the quality of AC Delco parts, particularly fuel pumps, has gone downhill over the years. I helped another member on here who had 4 brand new AC Delco fuel pump modules for his 97-98 fail withing 1 year. He finally gave up in changing them out and went with a Bosch module.

I'd connect a fuel pressure tester to your rail and if you don't have one some parts stores like Advance & Autozone have loaner ones. Spec pressure is 49psi +/- 5 psi and fuel pressure shouldn't drop below 24psi after 10-20 min after shut down or check valve on pump outlet is bad.

Since your main problems are on first start-up the pump should prime up to around 49psi on one key stroke. If it doesn't and the fuel line is empty the check valve is bad. The exception would be if you have real leaky injectors or an external fuel leak somewhere. But leaky injectors may just cause the rail to bleed off while the line should still be full of fuel.

The idle air control (IAC) can become pretty nasty and helps to smooth out the idle along with other things. Below is a write-up on how to clean it and remove the whole throttle body and clean it too. Just don't move or screw the pintel (tip) in/out while cleaning.

http://www.jeepforum.com/forum/f13/h...-valve-756934/

I would check your fuel pressure first to not confuse matters with 2 different parts possibly causing your problems. Good luck.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #7 of 9 Old 08-20-2017, 05:28 PM Thread Starter
jdhint86
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thanks for the advice uniblurb I'll check the pressure on the fuel rail this week but this is another piece of information I can give you guys on maybe helping me diagnose my problem I waited to start my zj until this afternoon today when the dew was off and I cycled the key a couple of times and it ran fine i have tried tge multiple key cycling in the morning before but did not help my problem so this maybe leads me to believe that the moisture is effecting something electrical but I'm not sure what else to check any suggestions
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post #8 of 9 Old 08-20-2017, 06:15 PM Thread Starter
jdhint86
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thanks I'll check the fuel pressure this week. I did try to start it for the first time today in the afternoon when the dew was off and it ran just fine. this makes be wonder if it's something electrical that the moisture is effecting but I've changed a lot of sensors is there anything else I should check.
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post #9 of 9 Old 08-21-2017, 03:01 AM
Uniblurb
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Since the wiring is under cover the dew shouldn't affect it but lots of dew usually mean high humidity. I'd make sure your fuel pump connector in front of the tank is tight together.

You could also make sure your horn works then flip the fuel pump relay with the horn relay.

Not sure if you used Mopar sensor or not when changing them out? Some of those aftermarket sensors, especially the crank sensor, creates even more havoc.

Lots of real good info in the dirty dozen thread in my signature. Look at the 'grounds' section and make sure all your grounds are good/clean including the PCM ground on one of the coil bracket studs. Best to access this ground from underneath.

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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