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Author Topic: 4.0 New Distributor Problems  (Read 2401 times)

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Brynjaminjones

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4.0 New Distributor Problems
« on: June 15, 2021, 07:50:28 PM »

Evening 'Dastards.

My Dad is having a bit of trouble with his 4.0 ZJ. I spotted whilst we were visiting that it had a pretty bad intermittent misfire and was running rich. He only drives it locally so I gave it a bit of a thrashing to try to clear it out, but after that it started knocking  :icon_eek:

I narrowed it down and fortunately found that it was the distributor. It had play in the shaft, causing the rotor to intermittently hit the contacts in the distributor cap and knock loudly.


I ordered him a replacement distributor, but I wasn't there when it arrived to fit it. Their mechanic has installed it and says it runs great at idle, but will backfire the moment you "give it the beans".
I'm guessing this is a timing issue - I figure that it could maybe have been inserted one tooth out. I've also read that the gear on new (cheap) distributors is sometimes 180 degrees out, meaning it won't go in in exactly the right position.

My theory is that it's therefore running too advanced, which it might like at idle but will go too far once you rev it, causing the backfire. :017:

Does that sound feasible to you lot, or can you think of anything else I should get them to look at? I hate trying to help them when I'm nowhere near - I'd happily tinker with it myself for hours, but I don't want them having to spend a fortune with somebody else doing the work.
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Dutch

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Re: 4.0 New Distributor Problems
« Reply #1 on: June 15, 2021, 08:04:13 PM »

Couldn't possibly comment on the technical aspects of your question.

If there is no conclusive answer here and your dad needs help please advise him to contact Mark Blessed - Blessed Motors in Treviscoe.

Mark used to work for Jeep Indian Queens until he set up his own business 6/7 years ago specializing in Jeep/Chrysler and general makes servicing from a double garage behind his house.  He's only got a Facebook page but all his contact details are on there.  Knowledgeable and very reasonable rates.

Mark has serviced the families fleet for years now and both kids bought cars off him.
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Brynjaminjones

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Re: 4.0 New Distributor Problems
« Reply #2 on: June 15, 2021, 10:40:12 PM »

Thank you! It's good to know, my parents don't really have a specialist to work on their Jeeps. We're also talking about moving back down someday so I'll remember that for when I get stuck!
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Jonny Jeep

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Re: 4.0 New Distributor Problems
« Reply #3 on: June 16, 2021, 10:29:44 AM »

Did you keep the sensor from the old distributor? Best to re-use the old sensor than use the sensor out of the new one, which can be kept for spare.

I'll post pics of the distributor alignment when home from work. My replacement distributor had the alignment pin in the 2.5 location rather than the 4.0 position, so that could be a factor.

More later...
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Brynjaminjones

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Re: 4.0 New Distributor Problems
« Reply #4 on: June 16, 2021, 02:33:18 PM »

I think they did a straight distributor swap so it would be using the new sensor. I'm trying to find out more but don't want to sound like I'm questioning their work!


They've now asked me to order a new CPS. I'm not convinced that's the issue as it seems a little coincidental, but I've ordered one anyway.
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Jonny Jeep

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Re: 4.0 New Distributor Problems
« Reply #5 on: June 16, 2021, 06:52:51 PM »

It's possible they inserted it with the pin in the wrong position which would mean the rotor would be significantly out from where it should be.

To ensure the distributor is aligned correctly you need to ensure the #1 cylinder is at TDC on the compression stroke. In my TJ the mount for the non A/C pulley gets in the way. I use the #2 with the plug removed and feel for compression build up as the engine is cranked around by hand using the crank pulley nut. Firing order being 1-5-3-6-2-4, I then have to turn the crank 2/3 of a revolution to bring it back to TDC at the end of the compression stroke. This is done to ensure the distributor isn't aligned with #1 when the #6 is at the top of the compression stroke.

Once the position of the crank is known, the distributor alignment can be checked. As mentioned, when I replaced mine the alignment pin was in the 2.5L position and I had to move it to the correct position. With the crank in the correct position the rotor arm and sensor should be removed and the alignment holes in the rotor plate and distributor housing should be inline if it has been installed in the correct position. See diagram below for alignment positions.

If it isn't aligned, removed the mounting bolt and hold down and pull the distributor.

With the alignment holes properly aligned, the distributor can be slid in place. The drive gear is helical cut, so when it meshes with the cam the distributor turns clockwise by about an hour. The distributor has to be started with the bolt down tab an hour back so that when it's inserted it will align with the hole in the block. I find it a bit tricky on the TJ to get the hold down bracket in place but once bolted down it should be good to go.

Edit: I'd ask them if they have the old sensor still anyway. Explain it's common knowledge in the Jeep world that aftermarket sensors can cause problems, and it's recommended to fit the original sensor to new distributors where possible.
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Brynjaminjones

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Re: 4.0 New Distributor Problems
« Reply #6 on: June 17, 2021, 10:17:31 AM »

Thank you, that's really helpful. I can't help but think that the problem is something similar to what you've described. Reading through the FSM has informed me that the distributor sensor is responsible for the fuel timing, not ignition timing. The backfiring seems more likely to be a fuel thing than ignition timing thing, so I really think the distributor must be the problem.


A new CPS is arriving tomorrow (RockAuto never cease to impress me!), but I'll wait to see if they reckon that works.
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