HELP - 97 New rebuild, stumbles when I give it gas (1 Viewer)

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Sep 28, 2014
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I got my engine back in after my rebuild, and it started right up. It idles great but when I give it gas it stumbles and backfires a bit, and wont rev past 1200 or so. If I slowly push the gas a little bit it will rev all the way up.

A little background - I've been slowly chipping away at my 97 restoration. just about everything new under the hood including brand new wiring harness. All parts painstakingly disassembled and cleaned. All rubber hoses replaced with new OEM. Tank pulled and completely de-rusted, you could eat off it. New Denso fuel pump. New fuel filter. Timing dead on 3° BTDC. New distributor wires, plugs, everything, all OEM. Fuel injectors serviced by witchhunter.

Fuel pressure relay tested ok. Fuel pump voltage reads hi and lo as it should at the fuel pump connector.

I thought it may be the TPS but when I pulled the plug on it it still had the problem, so thinking I can rule that out. I don't have a vacuum pump to test/adjust the TPS. I did remove it when rebuilding and didn't mark it's location so it is most likely out of adjustment. I've moved it a little here and there and it hasn't affected much.

I was getting some vacuum (or maybe pressure, couldn't tell for sure) in the fuel tank, probably due to the charcoal canister (not replaced) so I tried opening the gas cap. No change.

It seems to be fuel related, but I don't know for sure.

I'm stumped and need suggestions. Any help is greatly appreciated!
 
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ppc

M Go Blue
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You need to adjust the TPS per the FSM or alternately use the ODB2 interface, a Bluetooth dongle with an Android software app like Torque to make the adjustments. It is a very precise adjustment. Searching on "TPS adjustment" should provide a writeup of the method.
 
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Are coolant and knock sensors properly installed and harness attached?

Did you use any sealants on the threads of those items? If so, what?
 
Joined
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Messages
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You need to adjust the TPS per the FSM or alternately use the ODB2 interface, a Bluetooth dongle with an Android software app like Torque to make the adjustments. It is a very precise adjustment. Searching on "TPS adjustment" should provide a writeup of the method.
I figured that would be the case. I have read the FSM procedure, but I don't have a way to apply vacuum to do the test. Didn't realize it could be done with a dongle. Just ordered one for same day delivery so I'll give it a try today.

I'm assuming you just turn it until the sensor start to pick up throttle position?

Are coolant and knock sensors properly installed and harness attached?

Did you use any sealants on the threads of those items? If so, what?
They are installed and harness is connected. I used high temp thread sealant on most of these connections.
 
Joined
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Location
Olathe, KS, USA
 
I figured that would be the case. I have read the FSM procedure, but I don't have a way to apply vacuum to do the test. Didn't realize it could be done with a dongle. Just ordered one for same day delivery so I'll give it a try today.

I'm assuming you just turn it until the sensor start to pick up throttle position?



They are installed and harness is connected. I used high temp thread sealant on most of these connections.
I have not researched this but:

Is the sealant recommended per the FSM? I ask because frequently sensors as such use the contact of the threads with the block to use as a ground and it is a difference in voltage that causes the change in the sensor. I may be way off base, but it is worth checking into in case you have created an insulator between the sensor and the block.

Others more informed on the electrical side of things may chime in here about this.
 
Joined
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The FSM does indicate using it in some spots, but I did make adsumptions when installing it in others. I wish I could remember everywhere I used it but it was definitely anywhere that could leak. Not sure if I would have done it on the knock sensor since it is a blind hole. Easy enough to check though. I'll report back.
 
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Ok I got a dongle and hooked it up. TPS reads 9.8% at idle, 76.46% with pedal at the floor. MAF seems to be reading properly and increases with revs. Thoughts?
 
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I’ve set the tps with just a multi meter and follow the fsm and been successful, did you check your timing?
 
Joined
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I’ve set the tps with just a multi meter and follow the fsm and been successful, did you check your timing?
Timing is dead on. I will try to borrow a vac pump to do the fsm tps adjustment procedure but from what I’ve read it seems to be on target.
I set the timing per fsm procedure to 3deg.
 
Joined
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One thing to mention: the truck sat for 3 years before I got around to rebuilding it. The tank was a disaster. I completely derusted it and replaced the entire fuel pump assembly but I didn’t touch the lines. Could partially clogged fuel lines cause this?
 

ppc

M Go Blue
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Did you replace the fuel filter under the intake manifold?
Did you open up the main wiring harness and check for damaged wires where it passes the EGR?
 
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Just adjusted throttle pos sensor per fsm using vacuum to open the throttle opener and feeler gages. Everything is now confimred within spec. Eliminating tps as a possibility.
 

culturedredneck

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Sep 28, 2005
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How are your O2 sensors? Those symptoms sound just like what mine was doing when the sensors went bad. Sometimes they don’t throw a code.
 
Joined
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Finally some progress!

I jumped the fuel pump relay plug to bypass the fuel pump resistor and it revved up with authority!
I confirmed function of the relay and I have continuity through the resistor circuit to the pump. The relay is supposed to bypass the resistor on start up and under load. It clicks over on startup but not under load.

the cause of my issue seems to be that the command to switch to fuel pump hi is not happening when I press the gas. Now the question is what sensor/device tells the ecu to switch to fuel pump hi?

I’m so close. if anyone knows please chime in!
 
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