Hi - I've searched for hours but haven't found a conclusion to my problem.

I have a '94 Newmar motorhome I bought this spring w/ 55,000 miles and a 460 w/ Banks exhaust and Transcommand. Very happy with the power and shifting. Replaced it's fuel filter this spring.

It's problem is cold starts. If you just start cranking it seems it will never start - I've found if you give it about 4 sets of 3 second cranking it will finally pop to life ( a little bit of throttle opening seems to help ) and it runs rough for the first 5 seconds or so. After that it's fine.

When I turn on the key I can hear the electric pump run for about 1 or 2 seconds. I can cycle the pump like that 10 times and then go to crank and it won't work (thinking it was low fuel pressure that took a bit to pump up ). Only way it starts is to actually crank it for a bit.

It starts fine hot and doesn't stall or stumble so I'm thinking ignition and IAC etc are OK. I haven't checked fuel rail pressure but can do so if needed. I'm going on a 2 week trip with the family this August and don't want to leave something like this unattended in case it is something like a failing fuel pump or something that will cause havoc later.

 

If a little throttle helps it start, then that points to the IAC. Apparently they can stick.

When you crank, it should go to full open.

 

But if it was the IAC I would think it would have some issues with stalling or poor idle at other times, would it not?

Does anyone know the pressure I should see on the fuel rail if I were to test that?

 

IAC is only the long term responder for idle speed, ignition advance is primary, so perhaps you wouldn't. And maybe when the engine runs the vibration stops it from sticking. Don't know if tapping it before you try to start it would help.

Fuel pressure should be 39psi above whatever the manifold pressure is at the time. So you should see 39 with the engine stopped, and maybe 29 at idle.

 

Ok - I have an update that may shed some light on this for you smart guys.

I tested fuel pressure last night - it takes a few key cycles to get rail up to 33 psi. After pump stops pressure drops to 0 in a few seconds. Key on and it pops almost instantly back to 33. after starting it holds right at 33 and drops to 29 or so when you rev it up. goes to 37 for a second right when you turn off key.

I assume a check valve is not holding - are there any in the system other than on the pump? The vacuum operated regulator must have some way of holding pressure back from dumping to the tank return line?

I also checked the coolant temp sender at the front of the manifold - it reads 18k ohm stone cold and 10k when lukewarm. Any one know what it is supposed to read?

 

That's not good. 39psi is Ford's standard pressure for fuel injection. As I said, the vacuum hose attached to it is so the fuel pressure will be 39psi above whatever pressure is in the manifold. When you measure it, you're probably measuring relative to atmospheric (unless you had the other side of the pressure gauge also connected to the manifold).

With the engine stopped, the manifold pressure IS atmospheric. When the engine starts and idles, manifold pressure goes DOWN by roughly 10psi, and so fuel pressure should also go down.

When the engine speeds up, it takes more fuel, and so the pump has to supply more. If it doesn't keep up, pressure goes DOWN.

It sounds like you have 2 problems. One is that your regulator keeps the pressure too low, and the second is that your pump cannot keep the fuel coming fast enough (because the pressure drops when you rev the engine up, demanding more fuel).

If the pressure drops more after you let off the gas pedal after revving it up, this is OK, because this is when you see the lowest absolute manifold pressure.

It's not good because the fuel computer needs this pressure to be correct for the mixture to be correct. With low fuel pressure, the mixture will be lean. Normally these engines run fairly rich, and run in open loop control most of the time. (At least my 1994 Winnebago does). Your Banks exhaust system has already used up some of that margin of rich mixture, so low fuel pressure coul dbe hazardous to your engine's health.

Assuming you're not in a cold part of the world, that sounds about right. Have a look at Ford Fuel Injection Engine Coolant Temperature Sensor (ECT)

Michael

 

Thanks for all the great insight Michael.

I think I'll see if I can pinch off a rubber return hose somewhere after the pressure regulator and retest. Hopefully I'll find the pressures higher and the regulator is the culprit. Dropping that 75 gal tank ( 2/3 full ) is not going to be fun and the $300+ pump isn't something you do for the heck of it. The MH pulls fine if you get your foot into it, so I'm just not convinced of the fuel starvation concept from a bad pump just yet......

My 33 psi is on a Harbor Freight cheepy gauge, so I wouldn't be surprised if the real pressure is 5 or more psi different.

My engine coolant temp sensor can be taken off the 'list' it seems - though that table you linked to really confuses me. the output voltage seems it would be inverse to the resistance, not the way they show it. I see they call it a thermal 'transistor' not 'resistor'. Resistor I get....... transistor, not so much.

my luck when I get this fixed my mileage will drop even more!

 

Sounds to my like the fuel system is draining back into the tank. And the prestart fuel pump run is not getting the job done. That's why short start attempts work, you run the pump longer because each time you turn the ignition on the pump runs. On my carbed 460, if you install a switch to put power to the rear terminal on the starter solenoid then the pump runs for as long as you hold the switch on. I however, do not know if the EFI system works the same way or does the ECU control the pump exclusively?

 

I guess I ought to check my own fuel pressure with my HF gauge.

I agree the wording on voltage at the ECT sensor is confusing.

Did you find the page for the IAC?
Ford Fuel Injection Idle Air Bypass (IAB)
The tests they list are not conclusive, but are probably worth running.

The fuel pump should run for about 1 second upon power up, and then shut off. It should then start up again when the engine is turned over, and keep running as long as the engine is turning. It's all controlled by the ECU. The ECU also has a way to check fuel pump electrical operation.

Do you have a code checker? This OBD I system can have codes set, but still not set the Check Engine Light, so it's worth having an easy way to check for codes. There are also several test sets you can run.

What mileage do you get at the moment?

I'm at 6.5mpg, but what got me into all this is that that doesn't change as you vary cruise speed all the way from 50mph to 65 mph.

Michael

 

I did some more testing last night. The hose connection at my gauge was dripping last time. This time I cranked it tight and got a whole different set of results ( amazing that a moderate drip could throw off the numbers so much ).

Fuel pressure is now reading 40 or 41# and holds pretty decent ( dropped 10# in an hour after I shut everything off ). I hooked onto the pressure regulator w/ my Mighty-Vac and at first when I applied full vacuum ( about 25mm hg or probably about 12 psi ) it dropped the fuel pressure to 30. Everytime after that it stayed at 40 no matter what, acting like the regulator is stuck. I applied a bit of pressure to the PR and I got it to raise the fuel pressure to 45#, but couldn't get it to pull down past 40 with vacuum.

I'll be ordering a new PR on Monday and report back.

MH gets about 6.5 or 7 mpg. Weighs 21,000# though.