Bad Fuel Pump??? (TBI)

I have a 74 cj5 with a chevy 350. Last year, I installed TBI fuel injection by AFI. Everything went smooth and it ran great. A few months ago, I was wheelin out in the desert and came back out to the road to head home. As it got up to highway speed, it started crappin out, like it was starving for fuel. I pulled over, shut it off, started it up and it ran fine the rest of the way home. I wheeled it several times in the subsequent months with no problem, until about 3 weeks ago. Again I was just cruising down the road and it started crappin out. Shut it off, re-started, ran fine for a bit, then same thing. I limped it back to camp this way, then towed it home. I changed the fuel filters, one inline BEFORE the pump and one near the throttle body. Drove it last night and it seems that if you really get on it, or have it going up hill with a load, it'll crap out. If you cruise and/or don't 'stomp' on it for any length of time, it's ok. As soon as it's uphill under load with increasing throttle, it starts crappin out. Let off the gas and it seems to 'recover' but not instantly...as if the pump has to 'catch up'.

So...I'm thinking fuel pump. Mine is an external pump, located on the right hand side frame, which is about 6-8" or so ABOVE the fuel tank outlet. The AFI instructions made a point of saying the pump needed to be gravity fed, and it should be mounted as close to the tank as possible and BELOW the outlet. That's just not possible with my setup, so I got it as close as I could. I've also spoken to others who say the external pumps go bad, especially if they have to PULL fuel rather than being FED fuel.

A couple questions. Do these symptoms seem to point to fuel pump? What is the easiest way to test this? I have bought a new jeep and want to get this fixed to sell. I don't mind putting a new pump in, but I don't want to spend money unnecessarily. Any suggestions would be greatly appreciated.

Thanks,
Steve

 

If you have replaced the filters and are still having an issue then that points toward the pump. But here is another possibility. Have you looked at the spray pattern on the injectors in the TB to see if they may be clogged? There should be a fine mist. If you see any large dropplets then the injectors are not working correctly.

Also is this the origional fuel tank? If so there may be sediment and crud from the tank that is clogging your filters agian.

As for the inline fuel pumps, they really need to be below the fuel level in the tank.

The inline pumps do not last long when they have to suck the fuel against gravity. They tend to overheat from the workload and can not use the fuel as a heat sink like in-tank pumps.

I mounted my pump on the frame rail above the skip plate mid jeep so that it is low but still protected. This keeps the pump down to almost the bottom of a stock fuel tank and aleviates this problem. Its a little farthur from the tank then recommended but after 2500 miles and 5 years it has not had any issues.

 

Points ignition or GM HEI or?

When you have a sudden "won't run" or "just stops" problem, it's almost always because of no fuel or no spark. Almost all other problems take time to develop.

Sounds like all your problems happen when there's low vacuum, ie low engine speed and throttle opened wide. First place I'd look on your TBI is the MAP (manifold air pressure) sensor. Do you have an OBD setup for the TBI? Have you checked for codes? With any kind of EFI, that's the first thing you typically do. AFI probably has some recommendations about OBD testers. If you are worried about fuel pressure, I'd expect you can put a fuel pressure gauge inline with the fuel line to test that.

 

Thanks for the responses. I don't have the ability to read the codes, but have access to someone who does. I was hoping to eliminate some things on my own before going that route (backwards I assume). The distributor is GM HEI and came with the AFI TBI kit.

I will try to rig a fuel pressure gauge somehow so I can see it when driving. One thing the AFI instructions emphasized as well is that the MAP sensor be connected to a full vacuum source and NOT a ported source. How do I tell the differenc. Mine is currently connected to a nipple at the base of the throttle body on the back (firewall) side. It's ALWAYS been that way, but today, I also saw on the AFI site that a 'rich' condition can be caused by the MAP sensor being connected to a ported vacuum. It DOES run rich...fumes up the garage REAL quick. It has done this since install.

Steve

 

Ported vacuum is blocked by the throttle plate at idle, so it goes to near zero at idle. Manifold vacuum will have high readings at idle.

 

Thanks Tim. I'm pretty sure I have the MAP connected properly, but that'll be an easy check with a gauge.

Steve

 

You say you replaced the filter before the pump, but did you check the sock on the fuel pickup? What you're describing is pretty common when you get crap or water floating around the bottom of the tank and it plugs the sock until your fuel demand drops off, then the junk falls away from the sock and the pickup works fine again for a while. I'd check that before throwing a new pump on it.

 

This is an external pump, not IN the tank so I don't think the sock applies here. The inline filter BEFORE the pump in one of those annodized housings with a cone shaped 'stone' like filter in it...VERY fine. While it had a little residue in it, it was far from plugged up. None the less, I cleaned it thoroughly and put it back. I am going to try and rig a fuel pressure gauge that I can see while running to find out what pressure I have. That will indicate whether I have a bad pump or not.

Thanks,
Steve

 

If you still have the stock fuel tank with the stock sender unit then you may have a strainer (sock) on the fuel pickup, my 72 has one. The fuel pressure on GM TBI systems is usually about 9-12 PSI and a good pump should make about 20 PSI if you block the return line ( only do this for a short period of time to test the pump). An even easier way to check the fuel system if you have access to a scanner is to road test your Jeep while monitoring the O2 voltage (provided your system has a oxygen sensor and a data line) on hard acceleration the O2 voltage should stay at about .9 volts, if you can duplicate the symptom and you see the O2 voltage dropping below .450 volts at wide open throttle that would indicate that the system is not getting enough fuel, however at idle and part throttle you should see the O2 voltage fluctuating above and below .450 volts.

 

Something no one has mentioned yet is if this system has a pressure regulator it could be acting up also and dumping the fuel pressure back to the tank. I'd check the other things mentioned first but something to keep in mind.... When you have the fuel pressure gauge hooked up shut the engine off and watch the gauge. If the pressure drops off fairly quickly that's a sign the regulator is bleeding off or the injectors are leaking. If you pinch off the return line and it still bleeds off then most likely the injectors are bleeding fuel into the cylinders or the pump is bleeding back to the tank. If the pressure quits bleeding off with the return line pinched off then most likely the regulator is bleeding off.

 

Thank you all very much for the replies. I now understand the 'sock' comment. I don't think I ever answered the question of stock tank or not...it is NOT a stock tank but I don't know what it is. It was installed when I bought it. The PO modified the rear of the jeep, raising the floor some and thus moving the tank up underneath further. It is only about a 10 gal tank and it had connections for both the fuel outlet and return lines already there. When I bought it, it was carbureted so the return line was just capped off at the tank. When I did the TBI, I used that for the return. Also, I think the regulator is somehow built into the throttle body. There was no separate regulator included with the kit but the pressure is obviously regulated somehow.

I will see about doing some of these tests this weekend. Again, I appreciate the help!

Steve

 

If its GM throttle body based, the pressure regulator is built into the TBI. It looks like a dome just behind the injectors on top of the TB. Its a spring loaded rubber diaphram that is set at the factory and is not adjustable. Also the GM TBI runs at 13 PSI, the pump needs to maintain that preasure even at WOT for your testing purposes.

 

Hudson described my TBI correctly. I didn't know where the regulator was, but assumed it was part of the throttle body. There IS that 'bulb' by the injectors...now I know. Also, AFI says fuel pressure should be between 10-15 so 13 seems good to me. I'm going to test this over the weekend. The fuel pump sounds the same as it ALWAYS has when I first turn the key...it runs for a couple seconds then shuts off. I can't hear it when the Chevy 350 is pouring exhaust through the headers!

Steve

 

Ok first let me just say that this forum is AWESOME!!! Thank you all for responding to my question and helping me get to the bottom of this. I tested the vacuum source for the MAP sensor and it is indeed connected correctly. Then I took the hood off, interjected a fuel pressure gauge into the fuel inlet, then zip tied, band-aided, duct taped a way for me to SEE the gauge while driving. Fuel pressure was within spec in the garage and I could not make it fail. So out on the road I went....down this short grade then back UP it....ON the gas. Sure enough...it ran great for a few seconds then started to stumble. At that moment, I could see the fuel pressure dropping. I stayed in the throttle and it just kept getting lower and lower, to about 4lbs. I let off and after a few seconds it started to recover and finally fully recovered to 13lbs.

So...I'm concluding fuel pump! Do you agree?

Thanks again for all the help!

Steve

 

Check the sock on the fuel pickup in the tank first. If it is getting clogged then it will exhibit the same symptoms. Would hate to spend that kinda $$$ before rulling out the cheap fixes. That will require you to pull the pickup out of the tank and inspect it. But its just time and not $$$. Also look for any kind of sediment that might be cloging he pickup while you have the tank open. After rulling that out then I would say you probably need a fuel pump.

 

Again, thanks to all. You caused me to look further and it turned out to be pretty easy. I was able to get 'into' my tank through the top by removing the gas cap housing from the tank. My tank is NOT stock and the filler is in the floor under the rear seat. Guess what I found. Three things. A small circlip, probably not an issue. A nickle sized piece of foil, like you'd find sealing a bottle of brake fluid etc, lying perfectly flat on the bottom of the tank an inch away from the fuel outlet to the pump. And lastly, a good sized ball bearing up on the right side 'shelf' of the tank. I'm HOPING that the foil is the culprit, being sucked against the fuel outlet when the pump is workin hard. I won't have a chance to test it for a couple days but I'm hoping that's it. My tank does not have a 'sock' or other filter inside it. Even if this is NOT the problem, these things needed to be OUT of the tank so thanks again for causing me to look further...and I didn't even have to drop the tank! I'll keep you posted.

Steve

 

Re: Bad Fuel Pump??? (TBI) - MORE INFO

Ok, after going through the tank and removing extraneous non essential debris from it (see earlier post), the problem persists. After testing, I shut it off to chat with a neighbor, and upon trying to restart 20 minutes later, slow crank, dead battery. Jumped it with my TJ and put her back in the garage. But that got me to thinking. When the jeep started 'failing' a few weeks back the night BEFORE, I had to have someone come jump start it to get it going. Next day, it failed as described in this thread. And when I thought back to the very FIRST time it failed, I believe that was a trip where I began to wonder in my starter was going bad because with a hot engine bay, it would turn VERY slowly, but would always start. The battery is not an Optima, but one like it and cost me about $170 bucks about a year ago. It has definitely been acting funny lately...slow cranking etc.

So...is it possible that I have a voltage or battery issue? My voltmeter shows 14+ volts under normal operating conditions. So before I replace the pump, I thought I'd check this avenue. I can easily swap in another battery to test but wanted to see if anyone had thoughts on this.

Steve

 

Low Voltage could absolutely be part or all of your issues. This info would've been very helpful to know in the earlier posts I'd put a different battery in there, check the charging system, and make sure the fuel pump is getting proper voltage. Frequently the cranking system will draw enough from a weak system to create issues in the other electronic systems, or if the charging system isn't putting out enough can cause exactly what you are describing under load. Many years ago I swapped some parts for an '89 Cherokee because the guy couldn't get it to start. He had a bad battery in it and was jumping it with a small battery charger. It would give enough juice to crank but not enough left over to run the ignition and fuel injection. When I went to pick it up, I put in a known good battery, started it up, and drove it on the trailer. This was after I asked him if he tried another battery before we did the deal. Needless to say he was a little unhappy. I run into many driveability issues caused by weak battery's or charging systems, or simply because of corroded cables or battery terminals. The electrical system MUST BE in full/good operating condition before trying to diagnose the electrical system. The good news is you found some issues that would most likely have caused problems down the road.

 

I would remove the inline filter that is installed between the pump and the fuel tank. It is difficult for the electric pumps to draw through a filter. It could be your problem. Do not worry about the screen in the tank affecting the pumps draw, it is much less restrictive.

 

Re: Bad Fuel Pump??? (TBI) - More info

Thanks again. Nik, I didn't mention the battery before cuz it just never clicked that THAT could be the problem too. However, I DID put a known good battery in last night and it still failed. So...after all this, I 'think' the pump is still the issue but not positive. I think I'm going to order a new one, remove the inline filter that it currently AHEAD of the pump, and mount that pump a little better so it is at or below the fuel level in the bottom of the tank. Right now it has to run up hill a bit, meaning the pump has to PULL the fuel, especially when the tank is not full. It has been this way since I did the TBI, but maybe the thing is just tired of suckin fuel!

The charging system 'seems' ok, but the only reason I say that is because the volt meter shows good 'charge' (14+ ) during normal operating. However, it does NOT seem like the battery is getting charged properly, as it is dead or dying and should NOT be. It is barely a year old and was a GOOD sealed, no leak (like optima) battery that cost 174 bucks. So maybe the charging system ISNT working properly.

One last comment. When it fails, under load or heavy acceleration, simply letting off the gas and 'coasting' will let the engine 'recover' after several seconds. I've always 'felt' that this was the pump catching back up once the fuel demand subsides. And also, as mentioned in my earlier posts, I DID prove that the fuel pressure drops to 3-4lbs during these failures, then creeps back up to normal once the fuel demand is diminished.

Steve