new engine with miss fire
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New User
Joined: Sep 2009
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From: Texas
new engine with miss fire
Ok to make a long story short my ’93 Bronco 5.8L has 200,000 miles on it and a head cracked, so instead of taking the time to replace the head I replaced the engine with a brand new crate motor.
Since I replaced the motor I’ve noticed an intermitting miss in the motor does it real good at 1500 rpm and sometimes at idle. No Codes, changed the cap, rotor, plugs, and wires. When you first start the it before the rpms drop it runs smooth as glass. If you disconnect the map sensor it runs at higher rpms and runs smooth as glass. I’ve taking it to two different mechanics and both noticed the problem, but were unable to determine a fix.
Tried swapping out the map sensor, had the injectors cleaned filters replaced flow tested, tried swapping out the pcm, and I’ve emptied entire multiple cans of carb cleaner looking for vacuum leaks
Any suggestions the only thing not stock on the motor is headers and I'm not sure the other motor didn't have the same miss.
Since I replaced the motor I’ve noticed an intermitting miss in the motor does it real good at 1500 rpm and sometimes at idle. No Codes, changed the cap, rotor, plugs, and wires. When you first start the it before the rpms drop it runs smooth as glass. If you disconnect the map sensor it runs at higher rpms and runs smooth as glass. I’ve taking it to two different mechanics and both noticed the problem, but were unable to determine a fix.
Tried swapping out the map sensor, had the injectors cleaned filters replaced flow tested, tried swapping out the pcm, and I’ve emptied entire multiple cans of carb cleaner looking for vacuum leaks
Any suggestions the only thing not stock on the motor is headers and I'm not sure the other motor didn't have the same miss.
Lead Driver
Joined: Sep 2002
Posts: 9,182
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From: Gateway to the West
So, it runs better with a vacuum leak.
New motor + old IAC and intake = filthy bypass air passages in the throttle body and/or a functioning but dirty IAC solenoid and valve assembly. Consider removing and cleaning the valve assembly with carb cleaner that is safe for factory coated throttle bodies. Make certain not to get the solenoid (electronic) portion of the assembly wet.
Originally Posted by psdnick1997
If you disconnect the map sensor it runs at higher rpms and runs smooth as glass.
Hotshot
Joined: Jul 2002
Posts: 14,285
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From: Bend,OR
also whats your timing set at? and how did you set the timing.
make sure plug wires 7 & 8 are not touching, these wires are known for having cross fire issues which could also be a possible cause. do as Grey said also.
what brand/type plugs are you running?
make sure plug wires 7 & 8 are not touching, these wires are known for having cross fire issues which could also be a possible cause. do as Grey said also.
what brand/type plugs are you running?
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From: Texas
Yeah it does run better with a vacuum leak, I forgot to mention that I did clean the intake throttle body IAC and valve assembly.
Timing was set at 10 deg with a snap on timing light. Spark plugs are Motorcraft I can’t remember the exact number, but I double-checked them.
I have not checked seven and eight spark plug wires; both Ford mechanics that messed with it couldn’t remember which wires couldn’t be near each other.
I will check the wires and let you know if it fixes the problem.
Also forgot to mention that the plugs show the engine to be running lean and it passed emissions test with flying colors.
Timing was set at 10 deg with a snap on timing light. Spark plugs are Motorcraft I can’t remember the exact number, but I double-checked them.
I have not checked seven and eight spark plug wires; both Ford mechanics that messed with it couldn’t remember which wires couldn’t be near each other.
I will check the wires and let you know if it fixes the problem.
Also forgot to mention that the plugs show the engine to be running lean and it passed emissions test with flying colors.
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New User
Joined: Sep 2009
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From: Texas
Ok, Seven and eight wires were touching, but no change in the engine miss. I did notice that when I hold the throttle steady the rpms drop and then come back up again. When held steady at 1500 with no load. Only going off of the dash gauge I would guess a 100-rpm loss. If I put a load on the motor the miss goes away and on occasion I can feel it when driving.
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Joined: Jul 2009
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i am willing to bet that it has nothing to do with spark but a fuel issue. putting old filthy electrical sensors on a new engine is only gonna do the same exact problem as before.
i have learned to deal with a lousy occasional missfire at idle and at 1500rpms steady with no load. i searched high and low and i could never find the problem, but i am placing all bets on an injector getting lazy from tons of milage. they don't last forever.
if i were you, i would pull every spark plug out and write on paper what cyl it goes to and what the condition of the plug is. (wet, blackened, whitened, shiny) take pictures for reference too. slight discoloration can show you that a cylinder is either lacking or being generous.
try it and tell us what you have found.
i have learned to deal with a lousy occasional missfire at idle and at 1500rpms steady with no load. i searched high and low and i could never find the problem, but i am placing all bets on an injector getting lazy from tons of milage. they don't last forever.
if i were you, i would pull every spark plug out and write on paper what cyl it goes to and what the condition of the plug is. (wet, blackened, whitened, shiny) take pictures for reference too. slight discoloration can show you that a cylinder is either lacking or being generous.
try it and tell us what you have found.
Hotshot
Joined: Jul 2002
Posts: 14,285
Likes: 12
From: Bend,OR
when you set the timing was the motor to temp and the spout connector removed? also go to www.fordfuelinjection.com and look up how to test your TPS. if its not giving the proper voltages it can cause issues. also was your throttle body idle screw adjusted at any time? if so it needs to be moved back to its original location, instructions for how to do this are also on the website.
is anything not hooked up by chance that uses vacum. are you still running the plastic vacum lines or have those been changed out for rubber vac hose? and you might also check your fuel pressure at the rail with a gauge. both with key on and while the motor is running.
is anything not hooked up by chance that uses vacum. are you still running the plastic vacum lines or have those been changed out for rubber vac hose? and you might also check your fuel pressure at the rail with a gauge. both with key on and while the motor is running.
