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Stumble/miss/ I can't figure it out.

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Old 09-13-2018, 09:23 PM
Mvuu
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Stumble/miss/ I can't figure it out.

Gents please help! 96 f250 7.5. Idle is rough with a regular noticible vibration. This gets worse when in gear and or with the a.c. on. She seems to run fine otherwise and the issue is not noticible at higher RPM. She seems to start hard at first also, usually going on the second crank. I have read until my eyes have crossed and can't find a solution. Also there are no KOEO codes.
I have changed the spark plugs, IAC, EGR, TPS, coil, distributor cap, new fuel pressure revulator, amd new fuel filter, plug wires check out MAP is good, checked and added ground straps, pcv checks out, new air filter, checked base timing with SPOUT out, and checked for advance with it in, checked plug wire routing and ensured no cross over, cleaned throttle bodies, no obvious vacuum leakes.... as you can see I have been stumped.
I checked fuel pressure at the rail, and I think that I might have found something. 3-5psa KOEO, and only built to 28 psi at idle. Could this cause my problem? If so, fuel pump or something else? What should I check? I have a duel tank set up but both tanks cause stumble miss issue.
If that's not the issue is it PIP, COM, or something else??

thanks in advance for the help!
 
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Old 09-14-2018, 01:11 AM
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Yeah, low fuel pressure is an issue.
Unplug and block the vacuum line to the Fuel Pressure Regulator to see if the fuel pressure increases any.
Sounds like a fuel pump issue, or blockage along the way….
Did you run the KOER ?
 
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Old 09-14-2018, 10:43 AM
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And it continues....
with the fuel pressure regulator vacuum line disconnected the pressure rises from 28 psi to 38 psi. Is this still low?

After running the KOER test I retrieved a 334 code. Thermactor????

interestingly, after multiple tests, the scanner only recognizes the truck as a 6cyl. I think this is a problem. Why would it do that, and can it point to the problem??
 
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Old 09-14-2018, 12:32 PM
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Pressure spec is 39.5 and these system often produce 42-45psi so it is a bit low. Does pressure always raise towards the no vacuum reading when vacuum is attached to the regulator and the throttle is stabbed sharply on an idling engine? If pressure drops at all on this test the punp is weak.

Does the pressure hold after the engine is shut off?
 
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Old 09-14-2018, 01:36 PM
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Pressure seems to remain after the engine is off. That should mean the injectors are ok, right? There does not seem to be an increase in pressure with throttle manipulation either short stabs or longer throttle opening. So im assuming that i have a weak pump.
 
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Old 09-14-2018, 04:18 PM
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Originally Posted by Mvuu View Post
Pressure seems to remain after the engine is off. That should mean the injectors are ok, right?
Yes.


Originally Posted by Mvuu View Post
There does not seem to be an increase in pressure with throttle manipulation either short stabs or longer throttle opening. So im assuming that i have a weak pump.
Yes, the pressure should jump immediately.
 
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Old 09-14-2018, 04:24 PM
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Thanks for the response!
as I start to chase this fuel pressure problem, do I need to only replace the pump in the tank I am using? I have also read about external pumps and selector valve problems. Can you provide info there? And finally, will the low pressure cause the stumble miss symptoms I'm experiencing?

additionally, any idea why my code reader thinks I only have 6 cylinders? Could the lack of cylinder recognition cause my stumble problem??

thanks again!
 
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Old 09-14-2018, 05:34 PM
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Your truck only has in-tank fuel pumps, no external pumps or switching valves at all. Yes low pressure can cause those problems or at least contribute to them.
The code reader thing could be a real problem or nothing at all. Are you getting the 11 or 111 all clear codes?
 
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Old 09-14-2018, 06:01 PM
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111 code, which is good. I'll get a fuel pump put in soon....oh joy!

I'll keep you posted.
thanks again!!
 
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Old 09-14-2018, 11:20 PM
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Originally Posted by Mvuu View Post
with the fuel pressure regulator vacuum line disconnected the pressure rises from 28 psi to 38 psi. Is this still low?
After running the KOER test I retrieved a 334 code. Thermactor????
Even though it is too low, 28 to 38 shows that your new FPR is working.... Enjoy replacing your Fuel Pump.....

334 - EGR closed valve voltage high;
This could mean your EGR-Valve is not closing all the way, and a potential contributor to a rough Idle and other issues.
Disconnect and plug the vacuum line to your EGR-Valve, warm up engine, run the KOER.
If the Code 334 is still there; Remove the EVP Sensor from the EGR-V, spray in some Throttle Body Cleaner into the EGR-V.
Using something that will fit down into the EVP hole, tap the EGR-V Rod to potentially close a stuck Valve.
Replace the EVP, run the KOER, if the Code 334 persist, test the EVP electrical characteristics.
 
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Old 09-14-2018, 11:24 PM
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That code has me puzzled... the EGR valve is new...so it shouldn't be stuck. How do I check the corresponding electronics? I assume the EVP is the electronic attachment on the EGR?
 
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Old 09-14-2018, 11:28 PM
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Originally Posted by Mvuu View Post
as I start to chase this fuel pressure problem, do I need to only replace the pump in the tank I am using?
If you have two tanks, check the fuel pressure for/using both tanks before you proceed with pump replacement.

 
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Old 09-14-2018, 11:37 PM
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Originally Posted by Mvuu View Post
That code has me puzzled... the EGR valve is new...so it shouldn't be stuck. How do I check the corresponding electronics? I assume the EVP is the electronic attachment on the EGR?
"I assume the EVP is the electronic attachment on the EGR?" Yes

Is your new EGR-V OEM?
Ok, I see the EGR-V is new.
Sometimes the replacement EGR-V rod is a different length from the original and the EVP is reading a partially open Valve and you will have to modify the length of the rod on the EVP to compensate for the potential extra length of the EGR-V rod.
Do this anyway to see if Vacuum is leaking through the EGR Vacuum Solenoid; Disconnect and plug the vacuum line to your EGR-Valve, warm up engine, run the KOER.
 
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Old 09-14-2018, 11:54 PM
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Did you get this code after you replaced your EGR-V ?
 
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Old 09-15-2018, 12:41 AM
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Originally Posted by Mvuu View Post
How do I check the corresponding electronics?
What colors are the wires to your EVP?
One thing you can do to check shaft length, is to loosen the EVP screws, put some shims in-between the sensor and the EGR-V housing to lift up the sensor rod from the EGR-V rod slightly, then check KOER.

 
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