Troubleshooting assistance - please help!

Hey all!

I really need help troubleshooting an issue my taco has been having recently. It’s a 97 V6 4x4 5speed.

At the tail end of particularly demanding drives (think: over an hour on winding mountain roads), I start to experience what I THINK are fuel cuts. I’ll start to feel that the engine is down on power, and when under load it will jerk/buck a few times, even out for a while (but still have power issues), then start jerking again. It does this repeatedly until you stop and give it 15 minutes to cool down. When it happens, every “jerk” feels like a sudden and total loss of power. No backfire or anything like that, just like the engine suddenly turns off for a few short bursts.

It happened for the first time in October last year on my way home from Pismo- and that wasn’t even an uphill drive, just a very long one (5 hours), and it didn’t start doing that until I was like a half hour from home.

Second time was on my way to (and from) Big Bear in December. It did fine on the trails, but started happening both when I arrived and when I left. On my way home, it actually started doing it at the beginning of my trip- the winding roads coming down the mountain. It also happened towards the last 45 minutes of the 2 hour drive, it was doing it constantly until I pulled over on the freeway. It was so bad that time I almost called a tow truck. Let it cool down for a half hour, then I was fine the rest of the way home.

Most recently it was on my way back from Ocotillo. Started happening, again, on the windy mountain roads. Stopped to fill up my gas and after that I was fine the whole way home.

I’ve checked the plugs, they’re fine. I replaced the fuel filter thinking that may have been it, no dice. The catalytic converter is brand new, PO had it replaced before he smogged it for sale.

I’m kind of thinking it may be the fuel pump, but I don’t want to waste money throwing parts at it. I tried reading the live data at home- fuel trims looked good (like 1.5-ish short and long term), and no misfires at idle. Unfortunately every time this has happened, I haven’t had my reader with me, so I don’t have any live data during the issue. I now carry it in my truck just in case, so hopefully I will soon.

This only happens on long or demanding drives. In town, on trails, daily commuting- absolutely zero issues. If I’m just in town or on a short commute, I can absolutely romp on it with no problem.

EDIT: Oh, I forgot. No check engine light, ever. And I know it works, because it comes on during startup, and it’s come on before when I’ve forgotten to reconnect a sensor. And before anyone says anything about timing- every single belt and pulley on this truck has been replaced in the past 2 months. Not a timing problem, mechanically at least.

 

Does sound like a fuel pump that is marginally starting to fail, at least based on symptoms. But like you said, hard to say for sure without reading data live. That being said, if it's a '97 and never had the fuel pump replaced before it's a ticking timebomb anyways that could leave you stranded off the grid and I doubt you are wasting money by replacing it even as a maintenance item. Worst case you get a trail spare that half-works to carry in your parts box. Good luck.

 

Thank you for chiming in, I appreciate it. I’ve only owned the truck for about 6 months, but from the other work I’ve done on it so far I’d say it’s highly unlikely that the pump has been replaced. So you’re right, now that I think of it, it’s probably not a bad idea anyway. I’ll order it and get ready to take off the bed.
I assume this is one of those parts where it’s “OEM or bust,” so I’ll also ready my wallet

 

Have checked spark plugs, plug wires, coils?

 

I haven’t personally replaced any of that but I did check the plugs when I was doing the valve covers. Plugs look damn near new. They’re Denso dual-ground plugs. Wires and coils… not sure how I’d diagnose an issue with any of that aside from just replacing them, which is kind of what I wanted to avoid doing.

 

Ok. Then you can check off plugs as good. Usually on the wire and coils look for any signs of possible arcing.

 

Hmmm, new plugs. Long distance trips, perhaps fuel pump and or maybe a coil?
Surprised no codes have been thrown

 

Visually they look ok, but I did read about someone else on here who shut off the lights and misted the air with water and saw actual arcs happening. I may give that a try.

I know right?? I’d be overjoyed if she’d just give me a code. Just nothing, even when it got really bad on my way back from big bear. I’m begging for it to give me SOMETHING to go off of lol.
I’m honestly thinking it’s the pump but I’m not ruling out the coils. They’re just so expensive

 

Well, I changed out the pump today. Not sure whether that was the issue since the problem was intermittent, but check this out.

This is powdered rust that came out of the old pre-filter sock. Before installing the new pump I cleaned out the tank best I could. Whatever rust is left isn’t going anywhere without a serious scrubbing, but I wouldn’t be at all surprised if this was the root of the problem. I’m kind of shocked it was getting any fuel at all. There was at least a quarter inch of this crap built up around the inner part of the filter sock. Could be the pump itself was totally fine but I replaced it anyway (with a Denso unit).

 

That’s crazy!! What did the inside of the tank look like?

 

It was fairly clean with some rusty spots, which I did my best to remove with a little success. I got enough of it to convince myself whatever I didn’t get wouldn’t break free that easily.

I think what caused it is when I ran some marvel mystery fuel system cleaner through the tank last year. Which, looking back on it- duh. It really explains everything, especially how well MMO is known to break up rust. So that’s probably why the tank was so clean when I looked at it. The MMO worked too well

Oh, and, I can finally confirm that the pump (or the rust buildup) was indeed the issue. Took a long trip up and down some steep grades over the weekend- zero issues. Truck runs like a champ now.