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Loss of power/backfire on acceleration


Danny G

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1988 Bronco II 2.9l with a4ld trans. No codes to reference from. Idles perfectly. Runs great under load for about 5 to 10 mins then loses all acceleration power and backfires if i dont baby the throttle. Ive replaced fuel tank and in tank pump, both filters, and tfi module. Im wondering i should be looking into fuel more or could this possibly be timing related? I dont have a timing light or a fuel pressure gauge that will connect to this fuel rail so im pretty much shooting in the dark either way.
 


franklin2

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You are going to have to buy some troubleshooting tools, or you will be wasting money on guessing. I see Harbor Freight has a fuel pressure test gauge for $31.00. And they have a kit on sale for $39.00. But I see Amazon has some black Friday deals cheaper than that.
 

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Pull the spark plugs and have a look at the tips to see if you are running lean or rich
Back fires means a cylinder misfired and its unburned air/fuel mix was dumped into exhaust manifold, its then ignited by the hot exhaust of the next cylinder the DIDN'T misfire

So its a misfire issue, not necessarily fuel related, but could be fuel or spark
There would be codes for lean or rich

"Runs great under load for about 5 to 10 mins then loses all acceleration power and backfires............"
Do you mean it runs well when COLD and then misfires after warmed up?

That could be failing ECT(engine coolant temp) sensor, it tells computer when to set the Choke and when to remove the Choke

I would expect a fuel pressure issue to cause an issue warm or cold, also throttle sensor or MAP sensor, temp wouldn't matter

TFI and Coil can become temp sensitive
 

Danny G

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You are going to have to buy some troubleshooting tools, or you will be wasting money on guessing. I see Harbor Freight has a fuel pressure test gauge for $31.00. And they have a kit on sale for $39.00. But I see Amazon has some black Friday deals cheaper than that.
Ok thats what I was guessing. Mainly made this post wondering if it was more likely something else than fuel that i may have skipped over checking. Is that a kit that you know has the correct fitting?
 

Danny G

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Pull the spark plugs and have a look at the tips to see if you are running lean or rich
Back fires means a cylinder misfired and its unburned air/fuel mix was dumped into exhaust manifold, its then ignited by the hot exhaust of the next cylinder the DIDN'T misfire

So its a misfire issue, not necessarily fuel related, but could be fuel or spark
There would be codes for lean or rich

"Runs great under load for about 5 to 10 mins then loses all acceleration power and backfires............"
Do you mean it runs well when COLD and then misfires after warmed up?

That could be failing ECT(engine coolant temp) sensor, it tells computer when to set the Choke and when to remove the Choke

I would expect a fuel pressure issue to cause an issue warm or cold, also throttle sensor or MAP sensor, temp wouldn't matter

TFI and Coil can become temp sensitive
Thats where im kinda at a loss. I can let it idle up to temp for 30 mins and still get that first few mins after its up to temp with power. Curious about the coolant temp sensor. Thats the one right at the front of the manifold by the tstat right?
 

Danny G

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It seems like warm or cold doesnt matter more of an under sustained load.
 

RonD

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Now it reads more like partially blocked exhaust, "warm or cold doesn't matter"
And no codes
Wouldn't be ECT sensor

Do a sanity test on computer first
Unplug the throttle sensor or MAP sensor, but just one of them
Start engine
See if computer sets the correct code, 2 digit code list here: https://www.therangerstation.com/tech/obd-i-diagnostic-trouble-code-dtc-applications-2-digit-ford-codes/

Put a vacuum gauge on warmed up engine
18-21" expected at idle
Raise RPMs to approx. 2,000 and hold there, just high steady RPMs don't need a tachometer
Watch vacuum
Should drop when throttle is opened but then climb back up above 15"
After its back up, watch for it to start to slowly drop, that's a clogged exhaust
(FYI, if vacuum starts to go higher, that's a clogged air intake)
 

rusty ol ranger

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Does it backfire out the exhaust or intake?

And is it an actual backfire (sounds like a shotgun) or more of a cough?
 

Danny G

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Finally able to get some more time working on it. Fuel pressure at idle warm is 33. Took some time to climb from about 29 to there though. Still no codes did sanity test on ecm and it threw correct codes.
 

Danny G

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Now it reads more like partially blocked exhaust, "warm or cold doesn't matter"
And no codes
Wouldn't be ECT sensor

Do a sanity test on computer first
Unplug the throttle sensor or MAP sensor, but just one of them
Start engine
See if computer sets the correct code, 2 digit code list here: https://www.therangerstation.com/tech/obd-i-diagnostic-trouble-code-dtc-applications-2-digit-ford-codes/

Put a vacuum gauge on warmed up engine
18-21" expected at idle
Raise RPMs to approx. 2,000 and hold there, just high steady RPMs don't need a tachometer
Watch vacuum
Should drop when throttle is opened but then climb back up above 15"
After its back up, watch for it to start to slowly drop, that's a clogged exhaust
(FYI, if vacuum starts to go higher, that's a clogged air intake)
Idle vacuum is about 17 at higher rpm ups it to the low 20s. When throttle is released theres a short spike followed by return to idle vacuum. Thats all normal correct?
 

rusty ol ranger

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Idle vacuum is about 17 at higher rpm ups it to the low 20s. When throttle is released theres a short spike followed by return to idle vacuum. Thats all normal correct?
I wouldnt worry about that.

i would worry about your fuel pressure though. Key on. Engine off, it should prime up to 40ish. Right away.

Wondering if you have a dying pump or a plugged filter
 

Danny G

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I wouldnt worry about that.

i would worry about your fuel pressure though. Key on. Engine off, it should prime up to 40ish. Right away.

Wondering if you have a dying pump or a plugged filter
I replaced filter not to long ago. I guess ill get an inline pump ordered i think they arent too expensive and that pretty much would rule out every fuel component as either replaced or verified working.
 

rusty ol ranger

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I replaced filter not to long ago. I guess ill get an inline pump ordered i think they arent too expensive and that pretty much would rule out every fuel component as either replaced or verified working.
Im not sure but 88 may have a lift pump in the tank too
 

franklin2

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Idle vacuum is about 17 at higher rpm ups it to the low 20s. When throttle is released theres a short spike followed by return to idle vacuum. Thats all normal correct?
Idle vacuum at 17 is low. You may indeed want to check for a blocked cat.
 

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