2006 mustang 4.0 timing issue

kaotikd · Jun 14, 2026

I recently did the complete timing job on my 2006 s197 4.0. I have done many of these before but never had this issue. The issue is it will not start if the serpentine belt is on. It chugs and spits like it’s out of time and won’t idle with belt on. Once the belt is removed it starts and runs good.
I took the valve covers and checked the timing was not off and the tools go in like they are supposed at TDC with no issues.
It seems like somewhere something is too tight or something. I can’t for the life of me think of what it maybe.
Any help would be appreciated.

roddy · Jun 15, 2026

Is your crankshaft pulley/harmonic damper seated properly, and tight? If not, installing the belt could push it out of alignment a bit, and cause issues with the crankshaft position sensor. Just thinking out loud...worth a check.
I'm going to have to do that job someday, and I'm not looking forward to it.

kaotikd · Jun 15, 2026

Yes the crank balancer is tight and torqued, I’m kind of thinking it may be a tensioner issue. The tensioners I got with the kit seem to be different to my old ones.I have got new parts that were bad before.
I ordered two new tensioners and we will see.
Thanks

Juice · Jun 15, 2026

Bad alternator.

kaotikd · Jun 15, 2026

The car ran fine before with the exception of the timing chain guide being broken and making a heck of a noise.
I checked all the accessories and nothing seems to be binding or stick. I even switched the belt tensioner with my other Mustang to see and still same issue. I read the timing could be off but it ran great without the belt and has great compression on all cylinders.
Thanks

Juice · Jun 15, 2026

Put the belt on.
Unplug the small plug on the back of the alternator. (This will disable the charging system)
Start it up.
Crystal ball says so.

kaotikd · Jun 15, 2026

Will give that a try for sure, if that’s the problem that’s easy.
Thanks

kaotikd · Jun 15, 2026

Unplugging the alternator had no change.

Kev555 · Jun 15, 2026

My guess is something to do with your bottom pulley setup if you've ruled out an electrical issue with the alternator. Make sure the bottom pulley is fully home and is the timing ring is on the right way around? if its the same Crank sensor setup as the GT. I don't know if this would the issue but is the aux belt is correctly routed and no pulleys running in reverse?

whitmanink · Jun 19, 2026

make sure everything that the belt touches spins freely ,,
check youre power steering pulley wheel , the a\c , and all the idle pulleys..
and make sure they are good to go ,
also make sure everything is in "plane" and straight on the pulleys (no walking)

and it also goes without saying,, make sure your belt route is correct

2006 mustang 4.0 timing issue

kaotikd

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roddy

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kaotikd

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Juice

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kaotikd

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Juice

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kaotikd

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kaotikd

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Kev555

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whitmanink

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