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Discussion Starter #1
Hey guys,

Having a problem with a rough idle when sitting at lights and stuff. Normally she idles right around like 800ish rpm but occasionally she will stutter down and drop to about 600 and sound almost like she wants to stall on me, but doesn't. It will happen sometimes every 10-15 seconds and sometimes maybe once every few minutes.


I have:
-Replaced TPS (motorcraft)
-Replaced IAC (motorcraft)
-Replaced Spark Plug Wires (ford racing)
-Replaced Rotor and Cap (msd)
-Replaced Spark Plugs (motorcraft)
-Replaced Ignition coil (motorcraft)



Where else should I be looking at this point? It was much worse before I did all the stuff listed above but it's definitely still there.

My fuel pump is whining at me; could this be the culprit? Maybe a vacuum leak somewhere?

Open to any suggestions as to where I should be looking next, considering I replaced basically all of the common culprits for a rough idle.
 

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Discussion Starter #2
I should add that at anything above idle it runs perfectly fine and smooth. I don't get any high speed misses, backfires, any surging, or anything. It's only when I'm sitting at a light or in traffic, completely stopped.
 

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I could maybe see fuel pump, a vacuum leak will typically make an engine rev higher at idle. Sounds odd but how clean is the throttle body ? Been a long time since I've messed with an efi truck, but if they have the holes in the throttle blades or a gap to allow a preset idle then being caked with carbon might effect that.
 

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Discussion Starter #4
Actually we took the throttle body off and completely cleaned it when we replaced the tps since the tps mounts on the bottom of it, figured we might as well.
 

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Try checking fuel pressure to rule that out since the pump is making noise, it could be loading up with fuel at idle. Don't see how if the pump is bad thou, unless it's internally regulated an surging pressure.
 

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firefighterrw - do you also notice your battery meter drop when this happens? my 94 does the same thing. RPM's will drop to ~500 (feels like the truck wants to stall), the battery meter drops, then it surges up to 1100-1200 before coming back down to normal. I've replaced TPS, IAC, plugs, wires, coil, rotor cap and arm (and all seriously needed to be replaced). But, I'm curious if yours only does this in D. If I shift to N or P at a light, it all stops. Shift into D and it starts again.
 

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Discussion Starter #8
firefighterrw - do you also notice your battery meter drop when this happens? my 94 does the same thing. RPM's will drop to ~500 (feels like the truck wants to stall), the battery meter drops, then it surges up to 1100-1200 before coming back down to normal. I've replaced TPS, IAC, plugs, wires, coil, rotor cap and arm (and all seriously needed to be replaced). But, I'm curious if yours only does this in D. If I shift to N or P at a light, it all stops. Shift into D and it starts again.


Not that I've noticed but I will keep a good look at it.

FWIW any time I've seen an engine do something like that and it included a battery level drop it's usually because the alternator wasn't spinning well enough, especially when you're turning the wheel hard.

My Explorer did that and turned out the harmonic balancer had separated. Bad stuff.

Anyways I would check your pulleys and belts, maybe your tensioner, make sure nothing is slipping just in case.
 

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Did you read the info thread about calibrating the supposedly 'non adjustable' TPS?
 

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That's what my 91 did untill I dropped a new file pump in it. After that I had no problems with it. Assuming that's what it was.
 

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Discussion Starter #11
Did you read the info thread about calibrating the supposedly 'non adjustable' TPS?


Completely admit that I did not, I too thought they were non-adjustable....mind pointing me in the right direction for that one?
 

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Discussion Starter #12
That's what my 91 did untill I dropped a new file pump in it. After that I had no problems with it. Assuming that's what it was.

Mine has been squealing and whistling at me anyways driving me nuts so I was thinking of doing it anyways. :rockon
 

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Eric
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Completely admit that I did not, I too thought they were non-adjustable....mind pointing me in the right direction for that one?
Not trying to be a "Nancy-butt-in" or come off like an ass, but, you only need to "calibrate" the TPS if it's outside of the max recommended spec of ~1.1V (mine's at 1.18V).

When you first start your truck, you need to keep your foot off the gas. The EEC reads the voltage the TPS is kicking back and uses that as the base setting every time you start up. For certain, pre-set, voltage increments above whatever that, initial, base voltage was, the EEC uses the signal to compute throttle position. If you had your foot on the throttle, it causes a higher base voltage reading and would screw up the EEC's calculations for driving operation. I think 2.8V above base is what the computer sees as WOT position. I don't remember, exactly, though.

So, yeah... the setting isn't super-critical and, really, only needs to be adjusted if it's out of spec. Again, not trying to be an ass. Just sayin'.

My $0.02,
Eric
 

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Not trying to be a "Nancy-butt-in" or come off like an ass, but, you only need to "calibrate" the TPS if it's outside of the max recommended spec of ~1.1V (mine's at 1.18V).

When you first start your truck, you need to keep your foot off the gas. The EEC reads the voltage the TPS is kicking back and uses that as the base setting every time you start up. For certain, pre-set, voltage increments above whatever that, initial, base voltage was, the EEC uses the signal to compute throttle position. If you had your foot on the throttle, it causes a higher base voltage reading and would screw up the EEC's calculations for driving operation. I think 2.8V above base is what the computer sees as WOT position. I don't remember, exactly, though.

So, yeah... the setting isn't super-critical and, really, only needs to be adjusted if it's out of spec. Again, not trying to be an ass. Just sayin'.

My $0.02,
Eric
Man can you believe this ass with his butting in?!? Haha j/k :beer
 

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Discussion Starter #15
How dare you politely add helpful information to the conversation! BRB SMASHING FOREHEAD INTO KEYBOARD RAAAAAAGE


Lol thanks man. Mine isn't out of spec so that probably isn't the issue.
 

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Don't know if your 94 is MAF but a few years ago I had the same symptoms. Turned out to be the filament on the MAF was coated in oil from the K&N filter I had installed. I cleaned the filaments with some MAF spray and went back to paper element filter and the problem went away.
 

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Discussion Starter #17
Nope it sure isn't, I think the 351 doesn't go to MAF until 95.
 
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