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Bronco II 1988 2.9 engine hesitation warm and cold start up rough running


IIBRONCOS

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Transmission
Automatic
Just recently I noticed at start up, a not as smooth as it used to be running engine. Once in a while it will start and be smooth as it warms, and sometimes it will start up fast
but feel rocky as it warms up. Even warm if I hit the gas quickly there is a hesitation before it revs up. It drives fine warm, but cold it is not smooth. Most engine parts were replaced about 20,000 miles ago. The 2 things that were not replaced are the TPS and IAC both which are original. I did clean the IAC 20,000 miles ago. It currently has about 63,000 original miles. Any ideas where to start would be appreciated.
 


franklin2

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1984
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Check the sparkplug wires and see if they are rubbing anywhere. Easy for them to rub against the firewall.
 

IIBRONCOS

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I checked the plug wires and found no rubbing. I removed all of the Autolite AP-105 spark plugs, and a couple had some wear at the tip but all looked to be normal.
I installed a new set of Autolite spark plugs AP-105, gapped at 44. I started the engine and it did not solve the rougher idle and slight hesitation when quickly accelerating in park. I put 2 different vacuum gauges on the engine, and one read 14.5 and the other was at 18. The needles were steady with a slight variation at idle. Any help is appreciated.
 

Uncle Gump

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4.0L SOHC/2.9L
2WD / 4WD
4WD
My credo
Lead follow or get out of my way
I always ask this when someone asks for help with diagnosis...

What's the fuel pressure?

It really is the simplest thing to check and is everything to the engines fuel delivery.
 

IIBRONCOS

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I will work on getting a fuel pressure tester. Thanks
 

IIBRONCOS

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Well I thought fuel pressure was an easy thing to check, but not so far. I rented a kit from Autozone and it was defective. I then ordered a kit only to find out it did not have the proper connections to fit the B2. I sent the kit back and am now going to take it in and have the fuel pressure checked. I did notice after cycling the pump a couple times, when I pressed on the schrader valve I did not get a strong stream of fuel coming out.
 

Uncle Gump

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Lead follow or get out of my way
Dang... that's a string of bad luck. I had to replace mine and bought this one.

 

IIBRONCOS

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Automatic
Thanks for the tip on the gauge. It would of been faster and easier for me just to replace the high pressure pump !!
 

IIBRONCOS

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Transmission
Automatic
Found someone to do a pressure test on the fuel system, and the gauge read 29 lbs. Ordering a high pressure pump tonight.
 

IIBRONCOS

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1988
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Engine Size
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Transmission
Automatic
Installed a new high pressure pump. It is putting out 39 lbs of pressure at the rail. The B2 ran real smooth today. Thanks Uncle Grump for the telepathic diagnosis.
 

Uncle Gump

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Vehicle Year
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Make / Model
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Engine Size
4.0L SOHC/2.9L
2WD / 4WD
4WD
My credo
Lead follow or get out of my way
Well that's good...

It's really just where ya have to start. If the fuel pressure is off... nothing else works as it should.
 

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