I'm stumped - '93 Ford E350 lurching under load (Motorhome)

rgnprof

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#1
Sorry this is long, but I have posted questions related to this issue in other RV forums - let me try to be thorough, but succinct as possible. I have a '93 Cobra Passport on a Ford E350 chassis with a 460 EFI motor, with 62,000 miles. The "first" problem was that the coach would just cut out after driving it for several hours (almost like it was vapor locking)- seemed to be worse when hot. After much reading and research, I troubleshot a bad fuel pump...

Replaced the fuel pump with an OEM Motorcraft pump and that problem has gone away. I also had the really loud, whirring idle speed control valve/motor problem and I replaced it with a BWD part and no more noise.

But, I have 'created' a new problem - the coach now lurches ONLY under load. Whenever I step on the gas - going up a hill, or even down a hill - the coach does not accelerate smoothly. If I back off the throttle some, it runs fine. Cruising at 65-70 on a straight flat stretch, it runs great! It also idles a little rough - more noticeably so when in gear...drop it in to neutral and idle evens out. I just returned from a 4000 mile trip and it did it the whole way - but other than that, I had no problems (even got between 8-9 mpg's). The engine has plenty of power - and even driving through the mountains in Wy and Montana it ran well. It would lurch slightly when the transmission dropped in to 2nd on the way up a steep grade, but once it started to even out up the hill, no lurching - would easily climb the steepest grades at 45, 50 55 mph. Generally, seems to run better with the OD off, and I had a friend suggest that the torque converter lock-up switch was not working properly. He suggested that when it did it again, touch the brakes while still accelerating and see if it stops lurching - tried that and it made no difference!

Here is what has been replaced:

1) Throttle position sensor (Motorcraft)
2) fuel pump and filter (Motorcraft)
3) idle speed control motor (Borg Warner)
4) ignition control module (fender mounted) (Motorcraft)
5) plugs (Motorcraft), wires (Belden from NAPA), cap, rotor (Autozone)
6) ignition coil (Motorcraft)
7) air filter
8) catalytic converter (Walker)

I changed the Transmission fluid at 60,000 miles and no particles - old fluid looked good. Other than this lurching on acceleration, coach runs great and I'm getting about as good gas mileage as can be expected! I have tried pulling codes several times and none are present - and I have troubleshot the EGR system and it seems to be functioning properly. The last thing I changed out was the TPS, and I really thought that was it - no such luck.

I have been tracking fuel pressure issues of late on the advice from another forum member - very helpful, but I'm still not positive about what is going on here.

He thought I was having fuel pressure problems, so I hooked up my pressure gauge and took it for a drive. KOEO pressure is 32 psi, at idle it is 28 psi and with hose pulled off the pressure regulator the pressure is 38 psi. At highway speeds and WOT I'm getting 36 psi. I wanted to see if pressure would drop when it started to lurch and it does not - the needle flutters some, but stays within 2 psi...when I let off the gas and coast, the pressure drops back to 30 psi.

Most think those pressures are low and at WOT it should be around 45 psi...I replaced the fuel pressure regulator with an extra one I had (ACDelco) and it made no difference - except I haven't been able to get it out and drive it yet...all other pressures are pretty much the same.

I am right now trying to figure out what to do about these pressure readings before I move to electrical/ignition stuff. Haynes indicates an engine running fuel pressure of 30-45 psi and Alldata says 30-40 psi - no real mention of what it should be at WOT.

I have been advised to just add an extra, inline fuel pump along the rail...I also received another new Motorcraft fuel pump today as a replacement, so I may pull the in-tank pump again. It's possible that something is wrong with that pump, or that the sock filter has clogged up (when I last replaced the fuel pump in April, I didn't empty all of the fuel, or clean the tank any).

Needless to say, I appreciate the help and I am still moving forward - I think!

ryan
 

rgnprof

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#2
I just checked pressures with return line pinched off - KOEO 34 psi and at idle 65 psi, with a fluttering needle...
 

billr

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#3
Have you connected a vacuum gauge you can observe while driving? Does this engine have a MAP/BARO sensor, or a MAF sensor?
 

rgnprof

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#4
I have not observed vacuum while driving...has a MAP sensor only. I had previously checked the MAP using the procedure from fordfuelinjecion.com...seemed to perform within specs with a good vacuum supply...

I have checked vacuum while idling - within specs as well.
 

billr

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#5
What are vac readings at idle, in and out of gear? (and rpm's?)
 

rgnprof

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#6
Vacuum at idle in Park = 19"
Vacuum at idle in Gear = 17.5"

Vacuum at 1500-2000 rpms (no tach so just guessing here) = immediately drops to 13" when I first touch gas pedal then bounces to 20-21"
 
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#7
You might look for corrosion at TPS connector. If you open up the pigtail for your old TPS and find corrosion there, you probably also have corrosion on the engine harness side.
 
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#9
Had a problem like this before with this era of Ford with this guy.



Surging under light loads, no error codes, but then I pulled the map sensor and set it on my workbench with ground and 14.5 V applied, hand vacuum pump to that nipple and scope to the output terminal. Can't remember which way, but when applying vacuum 0 to 20"/Hg the scoped output was a 95 to 155 Hz output, while changing the vacuum severe jigger occurred on the scope. Got a new one and frequency changed smoothly and proportional with that vacuum change. Put that in and solved that problem.

Can't say this is your problem, but sure sounds like it. This almost drove me nuts, LOL, maybe it did.
 
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#10
Sometimes Ford connector grease looks like blue-green in color, and you think it always looked that way when in fact it is infused with color from adjacent copper wires.

If your old TPS has a pigtail, and you still have the TPS, it's real easy to open the harness to look. More difficult would be to do the same to the engine harness. A problem there is actually more likely, since the TPS may have been changed before, but the engine harness is probably original.

If the old TBS is original and corroded, then you should take the next step and open up the engine harness. But no corrosion found in the TPS does not insure the the engine harness wires are ok.

When you say "didn't notice any corrosion", that doesn't close the door for me that there is no problem there. On the other hand, from your previous postings, I seems to me that you DO know what the hell you are doing! It's just that corrosion at this spot is easy to miss.
 
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#11
LOL in regards to what Danica is saying, tell my own kids, you call those terminals clean, get down and give me 50, a hangover from my military days. A micron of oxide is all that it takes in these high impedance microelectronic circuits to cause problems.
 

billr

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#12
NickD, that was a MAP sensor you posted a picture of, right? The symptoms have had me suspicious of that all along, pay close attention to electrical and vacuum connections to it. I don't like to just throw parts at a problem, but it may be worthwhile to change the MAP, since this is OBD1 with limited live-data help and I doubt you have a scope to check it as a generic device. And let's not ignore the fact that NickD seems to have been down this same road; personal experience counts...
 

rgnprof

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#13
I do have the old TPS and the leads on it and the harness look good. I want to try and test it by backprobing the leads and moving the throttle, but I'm not exactly sure how to do this (and, it is really a pain to get to this thing on this van motor).

So, are we ruling out fuel pressure as the culprit? I am still real confused about the proper fuel pressure for this vehicle...And, shouldn't the fuel pressure stay the same throughout the throttle? Isn't that what the fuel pressure regulator does? Or does fuel pressure change based on load? And, one thing I think I have been overlooking - KOEO is only 32 psi and drops slowly as I leave the key on - Alldata indicates KOEO pressure should be between 35-45 psi.

The MAP is about $100 part, but very easy to change...I may try this next. Should I only buy a Motorcraft one?

ryan
 

billr

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#14
Fuel pressure, that you will read at the fuel rail, does not stay constant; it varies with the manifold pressure. That's not to say fuel pressure isn't your problem, but if the engine will pull a steady load then not so likely. One thing that always confuses me is "KOEO". Is that key-on engine-on, or key-on engine-off? For electrical components (like MAP) more and more people here are recommending using OEM (Ford, in this case).
 
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#15
Hate to have you spend a hundred bucks on my account and that not be the problem, electronic parts are not returnable. MAP also has three electrical connectors, 12V, ground, and output that have to be good. Plus a reliable vacuum hose and source. Wouldn't hurt to connect a vacuum gauge first. While a vacuum gauge show a steady reading due to an extremely small restrictor, manifold is nothing like that, but a series of sharp pulses, vacuum gauges show average pressure.

Could be when you step on the gas, higher combustion pressures can cause a leak back on the intake valves for one example, loose TB vane causes those problems as well sending wrong vacuum information to the MAP. Didn't mention I do compression test first, decarbonized the engine, that helped a little, but my MAP sensor was verified as defective. I did open it up, somebody at Ford forgot to solder a connection. But since this thing was all epoxied together, wrecked it taking it apart, so had to buy a new one, back then it was 60 bucks. Inflation.