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Need Help with 91, 7.5L that won't pass smog


2nd Ranger

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I've got a 91F250 that is driving me crazy.
New spark plugs
New Rotor
Wires OK
New O2 sensor
EGR clear, new gaskets to upper manifold and throttle body.
Idle air checked and cleaned
New EGR vacuum regulator
Found leaking line to secondary air bypass and repaired.
Secondary Air bypass solenoid works correctly as does the secondary air bypass valve.

Will pass smog if vacuum is applied to EGR valve so CAT OK
High NOX without EGR in the on position.

No DPEF sensor on this truck
Checked old MAP sensor and temporary swapped a new one, no difference.
Temporary swapped a new EGR position sensor, no difference, reinstalled old one.

Not sure what feedback loops turn on Vacuum to EGR but that's my only known issue now preventing passing smog. Did not replace the control module between wires and solenoid. Does that part fail?

Could I have low voltage from the ignition to one of the circuits not needed to start and run the truck? Chime is very faint with key in and door open.
Cruise control does not work. Rear fuel tank won't feed properly. Front Fuel gauge won't read properly.


I have read it could be the PCM.


Looking for advice on possible causes I've missed.

Thanks for any help you might be able to give me tracing this problem. 2nd Ranger out....
 


2nd Ranger

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A little more information

I unplugged the EGR solenoid and switch. With engine on I have approximately 14 volts across the leads power to ground side. When I plug in the switch, voltage across leads is about 11.2 or 11.3 at idle but will jump to 13.6 to 14 volts transiently when I stomp on the gas. Thinking computer is OK and MAF OK. Resistance across leads of old and new solenoid are 045 for the new one and 044 for the old one. Neither holds vacuum out of the car on either vacuum nipple. Wondering if I ruined the new solenoid by switching the vacuum lines. When plugged in and engine running, the solenoid will not hold vacuum. Could have gotten a defective switch from the auto parts.

Thanks again for any suggestions.
 

RonD

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Welcome to TRS :)

EGR systems main purpose is to lower Cylinder temps when engine is under load.
NOX gases spike high when cylinder temps get high

The exhaust gases slow down the burn rate of Rich air:fuel mix in the cylinders so keeps temp down a bit and less NOX is produced.

At idle EGR valve would be closed, on acceleration it would start to open as engine load increases, when cruising along it would open a bit depending on engine load.

PCM controls the EGR valve using the EGR solenoid, it is a vacuum control valve, it gets 12volts with Key on from the PCM Relay, same as fuel injectors, IAC Valve, ignition system and PCM does.
PCM Controls the EGR solenoid by Pulsing its Ground wire, so with key on engine off one wire will be 12v and the other nothing, if disconnected, if connected both wire would show 12v because its a Solenoid, like a relay, it is just a coil of wire inside so 12v in = 12v out, if Grounded then it becomes an electro-magnet and pulls open the valve.

If Ground is pulsed then valve opens a little or alot, depending on Pulse width.
Hard to measure this voltage because EGR solenoid is only used after warm up and then only under a load
System voltage is 13.5v after warm up engine running, if you hooked up a Volt Meter to BOTH wires, you should see 0 volts or very low volts, at idle, try REVing engine and leave it at 2,000RPMs, see what voltage you get, not sure it will change much if you have a MAP sensor.
MAP sensor reads intake vacuum, which is a way to read engine Load, throttle position and intake vacuum = engine load
MAF sensor and throttle position can also be used for engine load

There will be an EGR sensor, on later models DPFE was used, earlier used a sensor on the EGR Valve itself, it told computer if EGR valve was moving, opening, and by how much.
These were not very accurate because they didn't show the "flow" of exhaust into the intake, just the EGR valves position, so over time if(WHEN) EGR tube or valve got carbon buildup the flow would be less even though valves open position was the same.

DPFE sensor measures FLOW in the EGR tube, so PCM can get accurate flow rates as EGR tube or valve got carbon build up.

But the EGR solenoid, EGR valve and PCM were the same, just the sensors were different.

The spike in NOX means cylinder temps are spiking which could be spark retard/advance issue, or EGR flow is less than it should be, possible sensor issue or limited flow in EGR tube.


Pinging/knocking is also a sign of high cylinder temps, and does cause a spike in NOX as well, pinging makes a cylinder supper HOT.
 

2nd Ranger

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Ron, thanks for your detailed answer. I replaced the new solenoid with another after doing a test to the battery with jumpers and listening for a click. Nothing was happening. I don't think the first BDW solenoid I bought was working right. Now with a T into the EGR valve vacuum line I get near zero at idle and at cruise but it will spike up to about five to 10 mmHG for a short period of time under load and then drop back down as I get to cruising speed. Does this sound right? Smog guy was nice enough to do a pre check but I don't want to abuse his generosity. Nox will drop low enough to pass if we apply vacuum at 15 and 25 mph during the dyno test but I don't know if it will be that open on it's own. I had pulled and cleaned the valve and intake manifold/throttle body so I'm sure passages from the tube are OK.
 

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