98 MUSTANG BACKFIRES WHEN TRYING TO START

rob32188 · Feb 9, 2010

I'VE GOT A 98 GT WITH A 2004 GT MOTOR I RECENTLY INSTALLED.ITS GOT COMP CAM XE STAGE 2 CAMS THAT I SWAPPED FROM MY OLD ENGINE.THE CAR RAN FINE PRIOR TO THE ENGINE SWAP AND IM PRETTY SURE THAT I GOT IT TIMED RITE AS I TIMED MY NPI MOTOR WITH NO TROUBLE. WHEN I CRANK THE CAR IT TRYS TO START AND BACKFIRES. THE COMPUTER WAS TUNED FOR THE NPI MOTOR WITH THE CAMS AND OTHER BOLT ONS I HAVE INSTALLED. WOULD THE COMPUTER MAKE THE CAR BACKFIRE SINCE IT HASNT BEEN TUNED FOR THE PI MOTOR SWAP OR WHAT COULD CAUSE THIS ANY HELP WOULD BE GREATLY APPRECIATED.

wmburns · Feb 9, 2010

It is most likely backfiring because it is out of time in some form or another.

The cams are not in sync with the crank.
The cams are out of sync with each other.
The spark plug wires installed on the wrong cylinder. Most likely such a mistake would be made at the coil pack side.
The cams themselves have been swapped right/left.

The PCM differences between NPI and PI has to do with the timing curves (initial timing is the same).

If the crank position sensor were bad, the motor would not start.

A bad cam position sensor (CMP) will not cause a no-start. It can cause problems running including backfire. Check the wiring to the cam position sensor.

For an exhaust back-fire, think unburnt fuel (no spark, injector stuck open).

Please describe how you timed the motor.

If you need additional verification, perform a cranking compression test and a leak down test.

rob32188 · Feb 9, 2010

I put the engine at tdc,folded the chains and marked the two copper links lined one mark up with the mark on the crank gear and the other up with the mark on the cam gear,is there any way to check the cam sensor other than just replacing it? if the crank sensor was bad would the car try to start?

wmburns · Feb 9, 2010

What about the spark plug wire angle? Could be real easy to get a couple of wires swapped.

How did you index the cams themselves? How did you install the cam gear on the cam shaft?

Are you positive that the left and right cams have not been swapped?

>>From Ford service CD under engine assembly:
Step 30:
Rotate the RH camshaft sprocket until the timing mark is approximately at the 11 o'clock position. Rotate the LH camshaft sprocket until the timing mark is approximately at the 12 o'clock position.

rob32188 · Feb 9, 2010

Yeah I'm sure they didn't get swapped as i changed one at time from my old motor. The cams have a slotted spot on them for the cam gears i installed them the same way i did on the npi motor and it ran fine the first time i just can't seem to get it figured out this time lol.Your reply says step 30 is there a how to type of forum on stangnet that shows how to install cams so i can double check and make sure i've done it rite?

rob32188 · Feb 12, 2010

TTT ANYONE HAVE ANY IDEALS?

hotdog71 · Feb 12, 2010

Hey,

My 03 would backfire on start up or not catch when cracked and after a week at the dealers it turned out the Alternator had a fooked circuit. They told me the miss fire was caused because the alternator was not providing the proper voltage to all the coil packs and thus was backfiring or missing on start up. New part all good now!

It also had a bad cam sensor.

Does it fire up if you put your foot on the gas peddle?

wmburns · Feb 12, 2010

hotdog71 said:
Hey,

My 03 would backfire on start up or not catch when cracked and after a week at the dealers it turned out the Alternator had a fooked circuit. They told me the miss fire was caused because the alternator was not providing the proper voltage to all the coil packs and thus was backfiring or missing on start up. New part all good now!

It also had a bad cam sensor.

Does it fire up if you put your foot on the gas peddle?

Calling GDawg! Possible bad alternator rectifier diode.

rob32188 · Feb 12, 2010

No it dosn't make a difference, I've got a new alt. laying around I'll try that out thanks

89lxsport50 · Feb 12, 2010

hotdog71 said:
Hey,

My 03 would backfire on start up or not catch when cracked and after a week at the dealers it turned out the Alternator had a fooked circuit. They told me the miss fire was caused because the alternator was not providing the proper voltage to all the coil packs and thus was backfiring or missing on start up. New part all good now!

It also had a bad cam sensor.

Does it fire up if you put your foot on the gas peddle?

The same thing happened with my 02 alternator.

wmburns · Feb 12, 2010

Is the back-fire an exhaust or intake back-fire?

Check out the following link:

Back-fire - Wikipedia, the free encyclopedia

It also talks about a possible cause of back-fire being the valves not closing all the way. Which brings up two options. That something is holding the valves open or the valve timing is too retarded in relation to the crank (cam indexing).

For an exhaust back-fire, also think in terms of unburnt fuel in the exhaust. Ie no spark or stuck open fuel injector. Do you smell any gas?

Rickyll7 · Feb 12, 2010

rob32188 said:
TTT ANYONE HAVE ANY IDEALS?

You've already been given plenty of ideas and haven't reported back about most of them.

Double check your spark plug wiring... and let us know what you find.
Check your cam sensor and wiring... and let us know what you find.

Also, let us know what you find out when you try that other alternator you said you have.

Does it actually start? Does it run ok once started?

What about injectors... are you using the injectors from the old or new motor? If your using the new injectors, are you sure they are the same specifications?

What about the fuel rails... new ones or old ones? I hope you are using the old ones if your car was originaly a return system.

Are you sure you have the vacuum line connected to the fuel pressure regulator (return system) or pressure sensor (returnless system)?

Are you using new spark plugs?

I would go over the engine and take another really good look at your wiring and vacuum lines to make sure everything is connected properly.

Get out a hanes manual or something and double check your spark plug wiring.

EDIT: Also, are you using the new or old MAF sensor?

rob32188 · Feb 12, 2010

I have got a haynes manual and I've double checked the spark plug/coil pack timing, I had to use the npi fuel system to make it work in my car.Its an exhaust back fire,the car wont start at all it tries to but it backfires through the exhaust, I have installed a new cam sensor and it didn't change at all. I've got a new crank sensor ordered just waiting for it to come in.I have installed new plugs and wires also installed new msd coil packs and didnt make a difference, all vacuum lines are installed correctly according to haynes repair manual. I had a local shop check the cam/crank timing and it was correct.I'm using the old maf sensor because i was told it wouldnt' make a difference would that affect the car starting? You can smell gas after the cranking the car for a little bit.

Rickyll7 · Feb 12, 2010

rob32188 said:
I have got a haynes manual and I've double checked the spark plug/coil pack timing, I had to use the npi fuel system to make it work in my car.Its an exhaust back fire,the car wont start at all it tries to but it backfires through the exhaust, I have installed a new cam sensor and it didn't change at all. I've got a new crank sensor ordered just waiting for it to come in.I have installed new plugs and wires also installed new msd coil packs and didnt make a difference, all vacuum lines are installed correctly according to haynes repair manual. I had a local shop check the cam/crank timing and it was correct.I'm using the old maf sensor because i was told it wouldnt' make a difference would that affect the car starting? You can smell gas after the cranking the car for a little bit.

If your using the original pcm and tune, then you want to use the maf that went with that. So, yes, use the old maf.

So are you using the old injectors too?

Are you sure the new engine was running before you put it in? What did your local shop do to check that your cam timing is right?

You may want to do a compression and leak down test.

Oh... on the crank shaft, there is a gear that sends the engine speed signal to the crank sensor. You sure you installed that in there right?

rob32188 · Feb 12, 2010

I timed it and took it to the local ford shop to make sure i had it correct they checked it and said it was,yeah I'm sure the wheel is on rite because it's marked front and back and the motor ran great it came out of a 04 gt with 20k miles that was hit in the rear i heard it run before it was pulled and i know the cams are good because my car ran great prior to the pi motor swap.

rob32188 · Feb 17, 2010

Got it running today I installed the new alt., crank sensor,also installed different injectors. Its runs great and sounds even better.... I can tell a huge difference in the performance between the 2 motors, thanks for all the help.

Rickyll7 · Feb 18, 2010

Glad to hear it!

wmburns · Feb 18, 2010

What do you think was the root cause?

rob32188 · Feb 18, 2010

I think is was a weak crank sensor not 100% sure though because i had the alt. tested today and it was fine, also the fuel injectors looked to be in good shape none were stuck open.

98 MUSTANG BACKFIRES WHEN TRYING TO START

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