Fuel Pressure Questions

[Alan_nc]

94 3.0, auto
Engine runs fine but dies every now and then when you come to a stop.
Engine vacuum is 17-20

Borrowed a fuel pressure tester from Autozone.
KOEO 39-40 #
Engine running gage goes to 30#
At idle or when revved. Goes up and down maybe 1# as you rev it or return to idle.
Let engine idle for 15 minutes sat steady on 30#
Remove vacuum line from FPR - stays at 30# - engine does start to die.

Any suggestions?

 

[RonD]

Pulling off the FPR vacuum line can cause engine to stumble from the vacuum leak

Pre-1998 Rangers use 30-40psi fuel pressure, so yours checks out as OK
Pressure has to get down to 10-15psi for stalling

The IAC Valve is what the computer uses to set idle RPMs, might be time to pull it off and clean it
At cold start idle should be 1,000rpm or so, then as engine warms up computer will start to lower the idle by closing IAC valve

When you put Automatic into gear computer will bump up the idle using the IAC Valve, also when AC is activated, if so equipped

So IAC Valve is pretty important

In general warm idle in an automatic would be 750-800rpm, in Park, 800-850rpm in Gear

 

[Shran]

Changed the filter recently?

Should go UP when you crack the throttle (low vacuum allows more fuel through.) Your numbers indicate to me that the pump is probably working OK and the regulator is doing its job but there is a lack of fuel happening.

 

[Alan_nc]

Thanks for the reply:
Pulled the IAC and cleaned it. It does something strange. When you have the engine running and you pull the electrical connection off of it nothing particularly changes....but when you plug the connection back in the idle goes up for 5 or 10 seconds and then drops back down. Also I don't get any raise in RPM when I put the truck in drive. Has AC but it isn't working so I can't try that. Don't ever get that cold engine high idle that I should. Is there any way that I can do a positive check to see if the IAC is working?

I am concerned about bad gas and the fuel filter. The truck sat for almost a year prior to me getting it. It had a full tank of gas, I put a couple of cans of dry gas in and have run about half of it out but am thinking I should drain the rest of it and start with new.

Planning on changing the filter when I have all the old gas gone.

Anything else you can think of that I should do?

 

[Shran]

I'd do a new filter now anyways... gas that's only a year old wouldn't bother me much. In any case bad gas won't affect a filter much, it won't plug it up unless it's several years old or more and has turned to varnish - however if the filter is simply old and plugged up with dirt and grit, you could rule that out and either fix the issue or move on to other troubleshooting steps.

 

[Alan_nc]

Just went back to Autozone and bought a hand pump to pump the gas out of the tank and into 5 galloon can. The local hazard disposal place will take the old gas.
Also bought the fuel filter while I was there.


Still would like to know if there is a positive way of testing the IAC valve.

 

[RonD]

IAC Valve is working if RPMs go high when reconnecting it, as you described, and then go back down, computer sees IAC wires are unplugged so increase voltage for wide open IAC, so when you plug wires back in IAC Opens all the way
When computer detects reconnection it lowers IAC Voltage to establish idle again

So computer is setting the idle RPMs
Computer detects In Gear from the NSS(neutral safety switch) on the transmission, just above shifter

Should be a 4 or 5 wire connector, allows starting only in Park or Neutral, also runs the Reverse lights, unplug the connector and plug it back in to clean contacts

Also could be ECT(engine coolant temp) sensor is showing engine is warm all the time, so on cold start computer just thinks its a restart, engine is already warm, so no higher idle, and no Choke, so engine should struggle to run on cold start
Does that happens?

 

[Alan_nc]

I never get what I would call a "cold start condition". Starts and runs fairly well (no high RPM). Usually have to start it twice (first time it runs for about 30 sec and dies) second time it starts and just settles into normal slow idle.

Is there a way to check the ECT?

 

[RonD]

Yes, ECT can be tested with volt meter and sewing needle to pierce the wire

ECT gets 5 volts from computer on the grey/red wire
Other wire is the one you want to test
When engine is cold, key on, it should be about 3volts
Start engine
As it warms up voltage will drop, will drop to 1volt at about 150degF and then .6-.7 volt at 180-190degf, fully warmed up

If ECT is showing close to 1volt COLD then replace it

If you see a Red/White wire thats the wrong device, its the temp SENDER used for the dash board temp gauge, its a 12volt temp device

 

[Alan_nc]

O.K. Thanks. Will do the ECT check tomorrow.
Drained all of the gas (actually pumped it into a couple of cans). It looked pretty dark for regular gas.
Will put in the new fuel filter and a few gallons of new gas prior to starting it again.

Really appreciate you taking the time to follow this thread and answer my questions.

Alan

 

[Alan_nc]

Just did a voltage check on the ECT. Couldn't see colors so just checked both.

Key on - Engine not running: Engine stone cold, has been sitting all night.
2.95 volts on one of the wires
.095 volts on the other wire


By your description I need to replace the ECT.

 

[RonD]

No that looks right, the 2.95v

The other wire is reference voltage and can show as "ground" so on some meter will show low on others 5v, its a "pull up" circuit, used on most sensor to get accurate voltages

 

[Alan_nc]

Well I went ahead and pulled the ECT. Think I'll replace it at the risk of wasting a little money. Will also replace the thermostat while I'm at it (had to remove the housing to get the ECT out).

I know you mentioned that the IAC was doing what was expected but is there a specific test to make sure it is working correctly?

The good news is that with the fuel filter changed and new gas the truck starts the first time and continues to run (still no high idle at cold start).

 

[Alan_nc]

Put in the new ECT and thermostat. Don't think it helped any.
The truck is starting and running very well. I think mostly due to putting in fresh gas and a fuel filter.

Still no high idle when cold - it tries every now and then but won't keep it up.

 

[Alan_nc]

O.K. this is just weird. Truck sat all night. When I started it this morning it had a high idle for the first time since I've had it. Ran great. Idle settled down as the truck heated up.

Guess I'll just drive it for a bit and hope for the best.