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Generations 8-9 1987-1996 F150 + 1987-1997 F250, F350

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Old 01-13-2019, 09:40 AM
Kaajot Kaajot is offline
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Question Fuel Pump and MAP Sensor Ground Faults

Another stumper-

I cannot keep this truck running reliably. Every 2-3 months the system goes down to a rough idling, stalling, POS. It's a great truck if it would just run -- all new parts, etc. Restored minus needing some rust cut out on the rear fender wells now and replaced with a nice pair of guards.

I don't have the two fault codes (used a snap on, it read in English for us and it's -10 deg out so not in a rush to find my ODB blink tester!) and received two fault codes for fuel pump ground and MAP ground.

The truck is running rough. The MAP sensor is probably broken again -- we've warrantied/purchased 4 now in 2 years. Is there a good way to bench the MAP whenever I get this problem figured out to see if it's good or not?

Anyway, I benched the three relays with 12vs and checked -- got an audible signal on the continuity and you could hear the clicking of each relay closing its circuit, so the top part of my fuel pump/engine control is good. I've not detached the box to inspect the wiring harness on the back side yet.

I just replaced the fuel selector switch about two months ago. We had a no-start situation when that blew. Again, thinking a ground is shorting out all these relays and switches.

Fuel pumps WILL push fuel, but voltage is dropping. If at times I let my foot off the pump and the alternator/rpms gets low, then the truck begins spurting to stall. Sometimes I can recover with a rev, sometimes not. In order to get into gear or switch I'm doing the wrong thing and keeping the RPMS high (1500-2000 is my best guest minimal) in order to go from one gear to another (P-R, R-D, D-R, P-D). Switching gears causes the voltage to drop and stalls the car most times if not boosted with additional current from the alternator.

Battery is strong. It's getting abused just like the transmission, but it's less than a year old and should be working as intended. Always above 12.5V. Probably would be hanging above 13.0V if this problem wasn't sucking it dry.

MAP is offline and disconnected, but vacuum tube is still connected. The truck is shifting rough in drive from first through third. Fourth and OD are a little less bumpy, but overall it always has a "bump" or a rough hit at all transitions up gear. Down gear nothing noticeable happens, but a full stop is liable to wind up in a stall unless you break and add gas at the same time to keep voltage up (not a good idea, particularly in snow and ice right now).

I haven't checked the inertia switch yet -- believe I have one behind my right kick panel in the cab from what I've read? I guess testing that voltage would tell me if the ground is before or after it even if it is not faulty itself. Or it could be faulty? Since it just sends signal I think its more likely a ground out and not that component that's failed, plus I don't want to buy another switch for $50 if it's a ground. The last impact on the vehicle was a 12 pt buck in 2013 -- worked fine for years, so I don't think the inertia switch is going to be the problem, but any pointers on that or where else to look is appreciated.

I noticed in a 87 or 88 Ranger video on YouTube that they had voltage drops at a fuel pump and also lights flicker/dim and blink too fast. I do have some wiring harness issues on my tow and aftermarket license plate illuminator (custom bumper, required).

Does a 1992 F-150 4.9L (my vehicle, VIN Y) have a ground that's shared with tow, fuel pump relays, and map sensor?


PS: The truck often runs rich I think and a smell of rotten eggs/sulfur occurs occasionally. Believe it's just fuel pump controls going wonky, but wanted to throw in that symptom. No blue smoke out the tail pipe. White smoke is evident, but it's really eff'n cold and could be condensation?

Last edited by Kaajot; 01-13-2019 at 09:45 AM.
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Old 01-15-2019, 03:30 PM
Kaajot Kaajot is offline
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Did some more work on the truck.

No low voltage from the Fuel Relay to the Inertia Safety Switch. Removing and bypassing Switch works as advertised. Switch checked good.

We got into the fuel selector switch that was replaced about 6-8 weeks ago. Now, this switch (NAPA) should be good, so since we tried everything else and found no low voltage or broken wires that the only thing that could possibly be causing low voltage towards the fuel pumps would be the Fuel Selector Switch (2 Tanks) to the Fuel Pumps. The front appeared to be less dependable during this problem, so I opted to cut into the common ground a foot ahead of the fuel pump and splice a new ground directly to the battery. That accomplished, no change. Everything still showing same symptoms and yes, the front fuel pump still worked intermittently just as the rear fuel pump.

That lead us to the throttle position sensor and spacer. My friend decided he wanted to really probe the TPS and it is showing massive inconsistency of resistance. IT goes high, low, then high again. It should start at high and go to low as voltage is supposed to start around 1 (.99 factory, mine tested at .90) to top out at 5.0v (mine topped out at 4.6v). The resistance was not testing within the parameters of the throttle's opening and closing. At this time we think it's the TPS. The MAP also is still throwing codes and appears to have been shorted by whatever has been shorting the system.

We've a new MAP on order (replacement from NAPA under warranty) and a new TPS Motorcraft brand from RockAuto on the way.

I'm sure these two components will correct the truck back to high-mileage per gallon efficiency and smooth gear transition, but this problem has continually reared its head every 3-6 months or sooner. I'm assuming there's a ground that is shorting everything and causing damage to these sensors? Any ideas? It's definitely not the Inertia Safety Switch and that doesn't really tie into the MAP, so is there a common ground I should be looking for shorting between the MAP, TPS, and fuel selector (which would affect the fuel pumps).

Or should I bite the bullet and buy a new fuel pump for the midship (metal) tank and then get an aft one later? That'll space out $200 over a couple months. To my knowledge, each pump is 27 years old and has 165K miles on them now, so the fact they've not failed yet is amazing. I have had a lot of idle time too, so the actual pump time is likely closer to something around a 180-200K mile truck.

And that's my update after shooting wires all day yesterday.
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Old 01-18-2019, 01:36 PM
Kaajot Kaajot is offline
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Anyone with more experience in fuel pump ground fault code and throttle position sensor/MAP sensors continually shorting out got any ideas on the above posts?
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