Truck's a '93 with the 300 6cyl and a 5sp, all with around 230k miles.

After spending most of last spring/summer working on this truck getting it ready to be driven regularly after having sat for several years, I've finally got it going enough to be taking it on regular trips to my deer lease, roughly a 330 mile round trip. The last couple of trips, I've noticed that randomly, and without warning, the truck will lose power drastically while cruising at 65-70. RPMs will stay the same, it just feels like it lost ignition or lost fuel and the truck is coasting. I'll floor it, and nothing will change. No RPM increase and no power increase. I'll let off the gas for a second, then get back on it, and usually that will get it back to going. Occasionally I'll have to do that a couple of times, or even downshift into 4th due to loss of speed to get back going. It'll do it once, and then never again for the remainder of the roughly 2.5-3hr trip. And it always seems to do it once the tank is below half-full, and has never done it above half-full. I can't hear the engine over my tires (36" swampers make quite a howl at 70mph) so I can't tell if the engine is shutting off or just sputtering or what, but it loses all pulling power briefly, then will be fine. Tach will stay solid at 2400 rpm, and none of the gauges flutter or move in any weird way. It never happens on the trip out, but only seems to happen on the trip coming back at the end of the weekend. And it never at the same point along the way. The trips back are generally when I let the tank get below half-full, which seems to be the only correlation I have found. I generally fill up at or just before half-full on the way to the lease, and it's never happened. The first time it happened, I thought it was maybe just bad gas, but I've burned through a few more tanks, and it still happens, always once below half-full. Doesn't seem to matter if on a grade or on flat ground, and I can't make it happen or predict when it will. I'm not using cruise control, so I don't think that is related to the problem. It is also sometimes hard to start after stopping to fill up with the engine still warm, but always does eventually start. It never sets of the CEL, but I will check it for stored codes.

I know I have a vac leak somewhere, as indicated by my lack of AC or heat out the front vents at highway speed, although both work just fine. Floor and defrost work. I can also occasionally hear a hiss while shifting when off the gas pedal with the clutch in. So I will need to track that one down. I've read the writeups and seen videos on the HVAC vac controls and related problems...

I'm suspecting it's fuel-related, but that's just my WAG at the moment. I don't remember my exact fuel pressure numbers at the rail, but they were within the accepted range last time I checked. I will check it again though, just to be sure. I'm also going to spend some time with a vac gauge under the hood and see if I can narrow down any leaks, but last time I did this, everything seemed fine. But if anyone has any pointers in the right direction, I'm happy to hear them.

My searching on this has come up with a couple of related discussions, but those all seemed to involve either carbed engines or v8s, so they've given me things to look for, but nothing has been a definitive answer.

New parts list on this truck include:
New fuel sending unit. Then a new pump when the old "new" fuel pump quit working.
New fuel filter
New Motorcraft FPR (I have a new BW FPR that I can swap between, but both are good)
New coil
New autolite copper plugs with delco wires
New IAC valve
New idle air temp sensor
New engine coolant temp sensor
New MAP sensor
New TPS
New PCM
New AC complete system
New EGR tube
New O2 sensor
TFI tested good
New clutch, slave, master, and rear main seal
And a bunch of new gaskets (haven't done head gasket yet, but haven't seen a reason to yet)
New water pump, and all new hoses, but that shouldn't matter for this

 

Just a guess that something is wonky with the fuel pickup, as the issue only happens below half tank. Possibly mis-shapen, so that it doesn't like to pull fuel from the bottom of the tank like it should. If I remember correctly. the pump is down low, submersed in the fuel. Hard starting when warm could be the pump getting warm from not being submersed, not putting out pressure like it should. Just because a part is new doesn't mean it is good...

Might as well check battery terminals, grounds and so forth. Higher resistance in connections could cause some goofy issues as well.

 

 

Do you have dual tanks and if so does it do it on the other tank also? If you are only using one tank, why? Sandy

 

If it were electrical/electronic you would probably not “stay solid at 2400 rpm”
So, I’m guessing Fuel issue.
You need to extend and tape your Fuel Pressure Gauge to your windscreen and see what it reads when this happens.

“I know I have a vac leak somewhere”,
So, disconnect and cap the port where the HVAC gets its Vacuum from the intake.

Are you having any transmission issues ?
Differential Speed Sensor issue ?
Computer not allowing additional fuel?

 

Yes, it is submerged and picks up at the lowest point in the tank. I was thinking it could be a heat issue with the pump as well, but the thing that makes me question that is the fact it only happens once, and then never happens again. I'd expect it to happen multiple more times along the drive if it were heat buildup. But still something to consider...

Terminals are all good, I know for a fact. I recently installed some extra relays for lighting, so all battery and ground connections are solid.


I replaced my original sending unit with a spectra-brand unit. After the install, it was still showing signs of poor fuel pressure, and I found a small metal rivet in my filter, which I assume was from the "new" pump. I replaced that pump with a new delphi unit, which got my pressure back to where it should be. I'm assuming this pump is working as it should. If it isn't, I'll spend the extra for a motorcraft unit.

The tank was dropped, drained, and thoroughly cleaned out when I replaced the sending unit. I did it again when I had to drop it a second time to replace the pump. I'm confident there's nothing in the tank. It was spotless when I put it back up. The sock on my original pump was completely full of rusty debris. Probably why it was not working...


Single rear tank only.


No speed sensor issues, and no transmission issues. It's a manual, so there wouldn't be any shifting issues that weren't caused by me

I'm assuming the PCM has had enough time to calibrate itself correctly. It's a new (reman) unit, and it's made a couple 300+ mile trips, and dozens of around-town quick trips since plugging in, so I'd hope that would be enough time to figure things out.

 

Season closed last Friday, so that is your fix. Seriously, I do not see where you ran diagnostic codes. Since you have replaced many major components, have you gone back and checked them to make sure they are good. Bad parts are very common. Have you tested fuel injectors? Not sure but if it would help with the problem, but it wouldn't hurt to check your compression with the mileage you have. The half tank thing is really confusing as you know. Sandy

 

Nope, I'll just take my 4runner...

I haven't checked for codes yet, but will when I get more time to mess with it. I'm not throwing a CEL right now, so I'm assuming there won't be any codes. Components that I can check have been checked and proven to be working as best I can tell. I broke out my DVM and tested virtually everything electrical under the hood when I was working on it earlier in the year. But I will go over things again if needed. I had my injectors ultrasonic cleaned and flow-tested earlier this year. One was dead, so it was replaced, but the others tested fine after cleaning. I checked compression earlier this year or maybe last year as well, and wrote down the values, but don't have them handy, but there was nothing alarming from what I remember. I will check it again the next free weekend I get...

 

CEL doesn't need to be on for codes to be stored.

 

Do you have cruise? If so, are you using it when it loses power? Also when you run the codes like timbersteel suggested, post them and indicate which are KOEO KOER and CM. Sandy