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Stumbles on initial acceleration

Brian1973

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Have a problem that feels like an air leak. Previous intake tube was starting to rot... slight cracks in according section led to air leaks that caused similar issue. Replaced tube, everything ran fine for few months. Now the similar problem has developed and I'm wondering if it isn't something new causing similar issue. Don't hear any air leak and tube is secure. I'm thinking fuel pump? I have to turn key couple times to get good pressure I suppose... worsening problem but wonder if it could be affecting JUST that initial take off when torque is applied to the ground... in neutral it seems better. Take off from a stop and stumble, pop, pop, pop, leave off gas a little and slowly give it pedal and it does ok... switching manual gears and all ok when reving back up.... only when first taking off from a stop. faster I hit the acceleration pedal, the more the problem! ideas?
 


Fordblue213

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Sounds like low fuel pressure. Popping in the exhaust is a lean condition. Went through the same with my F150.

How long has it been since the fuel filter was changed? That's a much cheaper and faster job and should be done anyway if you end up having to do the fuel pump.
 

RonD

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Yes, +1 on the fuel filter, but I would expect that issue to also effect top speed.

Test fuel pressure if possible, failing pump OR FPR(fuel pressure regulator) may be at fault

Simple test for air/vacuum leak is to warm up the engine
Let it idle and unplug the IAC valve's 2 wire connector
Idle should drop to 500 or so, or engine may even stall, either is GOOD, it means no vacuum leak

If idle doesn't drop then start checking hoses


The TPS(throttle position sensor) does a few things for the computer
First is that it gives the computer a "heads up" that driver wants to increase RPMs, and it is instant, unlike MAF sensor detecting increased air flow a second or two later.
So stumble on acceleration could be TPS issue
Unplug connector and plug it back in, that cleans the 3 connections
 
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Brian1973

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Thanks for the ideas guys!

Fuel pump was changed about 10 yrs and 45k miles ago. At the time, shortly after new install, I blew the seal in the FPR and hydro-locked engine! :-( I did an engine swap, sigh. I believe I changed the filter when swapped the pump... possible I didn't, hmmm...

Unhooked connector to IAC and engine stalled. Good idea.

Unhooked the connector to the TPS a few times and nothing different happened when quickly pressing gas pedal. Pop, stumble, pop, pop!

I was wrong when said it will only do at initial start/acceleration. If rpms get low in any gear, it will pop and stumble.

I will work on getting a fuel pressure guage as I believe I can hook to a schrader valve on the fuel rail... Anyone know if Autozone has one of those in the loaner program? If not I know a mechanic I can probably borrow from. I guess a clogged fuel filter could cause a low pressure issue (turn key few times to charge)? Will check pressure before and after a filter change if go that route. But yeah, was thinking I'd have issues along all rpms if something was going on with the fuel pump.
 
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RonD

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Engine needs to be warmed up first for IAC Valve test or it will stall every time
 

Brian1973

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Thought it was warmed up enough last time, couple mins last time. Retried after 10 mins, idle dropped, no stall. :icon_thumby:
 

Brian1973

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Sooo, still trying to figure this issue. Part time truck so not trying real hard obviously... Problem seems to be progressing in that now after a long hwy drive, or rather after heated up pretty well, say going up a long hill on the hwy, it starts to stumble. Sort of like a miss. Get off the hwy and it behaves very badly compared to when first start. Idles fine but give it gas and it will stall if do not feather the gas just right with the clutch... backfires... Possible EGR valve issue? 200k miles. Possibly a couple issues at the same time? Hard to start points to Fuel Pump but the stalling seems to be something else? Fuel filter has been changed recent. No difference.
 

RonD

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Clean MAF sensor
 

Brian1973

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MAF cleaned, no help... new air filter.... did find a crack in hose going from intake tube to crankcase so replaced and that helped the severity of the issue but it still exists... it is the popping, stumbling at take off and also at low rpms below 3000... it is also max rpms that shows it self like a rev limiter on a race car is the only way I can explain it... power slides and the popping and crackling starts.

just hooked a fuel pressure tester to the schrader valve on the fuel rail and I am getting ZERO movement with the key turned or engine running. Trying to figure if I have it hooked wrong but it's pretty straight forward and the needle seems to just SLIGHTLY move when the key is turned.... really have to look hard to see it.... Used this same tester a couple years ago and I think I had a 30psi on the dial? no fuel leaks anywhere on tester... Can this truck run without pressure?!? Or is that my whole problem as I figured to begin with? Figured I'd have to have SOME pressure though?

Someone said something to me in person about a crankshaft sensor? or camshaft sensor? that can get hot and could cause issues... Severity does seem to increase after engine is hot! I replaced the ignition module/exciter? a few years ago, could that be going crazy again? I forget what the symptoms were before. I also locate no such camshaft or crankshaft sensor near the crank pulley... shall I try a cheaper option for now of the Fuel Pressure regulator vs. the pump? or start the process of dropping the tank? I'm thinking fuel pump if y'all think this truck can run on zero pressure... albeit, not very well.... if not, then there has to be a fault in my gauge or the tubing. (sigh)
 
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Brian1973

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Tried a different hose in fuel pressure tester kit, now I got some results... first turn of key, 10 psi and fast leak down, second turn of key, up to 15, 3rd turn, 20psi.... and immediate leak downs... Fuel pump, you are coming out!
 

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