Accelerating troubles on 3FE

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Mar 31, 2015
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I'm working on a problem with my 1991 3FE FJ80. It has started having problems accelerating and going up hills. I'd describe the problem as:
-press on the accelerator
-tachometer goes up to about 2800 rpms
-needle on the tac starts to waver at about 2800-2900 rpms
-engine at the same time starts to shudder
-tach starts to jump all over the place from about 2400 to 2900 rpms
-vehicle starts bucking and trying to accelerate. Like, trying really hard to accelerate but just bouncing between 2400 and 2900 rpms.
-acceleration eventually sorts itself out after taking foot off the accelerator and slowly accelerating up to speed

Idle is pretty good, it's about 650 rpms so it's where I want it.

I'm going down the troubleshooting list in the FSM. PCV valve, grommet and hose need to be done, and it's been ordered. But I don't think that explains this behaviour.

I have:
-run injector cleaner
-checked all air and vacuum hoses for leaks, put a bunch of new hose down
-replaced the fuel pump as well, and after replacing it the system was back to normal, but then I realized the system is always good for about the first 20 minutes of highway driving- accelerates and climbs hills well. Then after around 20 minutes of driving if I try to accelerate it'll just bog down and be unable to accelerate properly.

Any ideas? I'm thinking it's a fuel problem, but what's weird is that there doesn't seem to be a fuel problem for those first 20 minutes on the highway.
 

inkpot

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Sounds like your drive plates are stripping out. Common on early rigs. Do the symptoms go away when you drive it in low range?
 
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Sounds like your drive plates are stripping out. Common on early rigs. Do the symptoms go away when you drive it in low range?
I'm not sure I could replicate it at highway speeds that way. That's a key part I foolishly left out- this only usually happens between 50 and 60 mph, but I haven't tried the hills in low range. Sharp acceleration usually does it, and it has bogged down on hills going 30-40 mph
I'll have to give it a shot on a steep hill and see.
 
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I had something similar happen recently. It was the first time in nearly three decades that the 3FE has given me any trouble.

Returning home from a tour it began surging (losing and regaining power) at one second intervals between 2300 and 2900 RPM. This occured only under load and in fourth gear (on slight hills for example, before downshifting). I had to run in 3rd gear above 3100 RMP to avoid the problem getting home. Once in town it stalled at the first stop sign, then recovered. There was a heavy gas smell. I found evidence of oil in the air filter, suggesting blow by.

- For blow by, checked cylinders and walls - were fine
- Vacuum system smoke test - showed no leaks
- Cleaned fuel system
- Replaced fuel pump
- Replaced air filter

Problem has not shown again in 3000 miles. While it's apparently fixed, I can't pin the diagnosis to fuel system cleaning, fuel pump replacement, both, neither, or something else like bad fuel.
 
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When you changed the Fuel pump, did you install a new fuel sock/strainer on the bottom of the pump? If not do that and report back. I have seen this situation occur when the sock/strainer was not changed. I would also change the inline fuel filter on the frame as well.
 
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When you changed the Fuel pump, did you install a new fuel sock/strainer on the bottom of the pump? If not do that and report back. I have seen this situation occur when the sock/strainer was not changed. I would also change the inline fuel filter on the frame as well.
I did the sock, but didn't do the fuel filter. I read a bunch of stuff on here about it beimg some kind of forever filter or something...but it is on the list now.
 
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I had something similar happen recently. It was the first time in nearly three decades that the 3FE has given me any trouble.

Returning home from a tour it began surging (losing and regaining power) at one second intervals between 2300 and 2900 RPM. This occured only under load and in fourth gear (on slight hills for example, before downshifting). I had to run in 3rd gear above 3100 RMP to avoid the problem getting home. Once in town it stalled at the first stop sign, then recovered. There was a heavy gas smell. I found evidence of oil in the air filter, suggesting blow by.

- For blow by, checked cylinders and walls - were fine
- Vacuum system smoke test - showed no leaks
- Cleaned fuel system
- Replaced fuel pump
- Replaced air filter

Problem has not shown again in 3000 miles. While it's apparently fixed, I can't pin the diagnosis to fuel system cleaning, fuel pump replacement, both, neither, or something else like bad fuel.

This helps a lot. I need to clean to fuel system. I'm not familiar with blow by, but I'll have to check into that since there was some oil in the air cleaner hose.
 
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Is this vehicle speed dependent, engine speed dependent, throttle speed dependent, or load dependent?
Feels like it is mostly load dependent. It has everything to do with how hard I'm asking the machine to accelerate based off how much gas I'm giving it.
 
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This helps a lot. I need to clean to fuel system. I'm not familiar with blow by, but I'll have to check into that since there was some oil in the air cleaner hose.

@jonheld might have some insights. He's the 80 series 3FE expert on the forum, and helps many think through troubles. Watching; please update the thread of your progress, and good luck.
 
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This helps a lot. I need to clean to fuel system. I'm not familiar with blow by, but I'll have to check into that since there was some oil in the air cleaner hose.

For what it's worth, my in line fuel filter and fuel sock / strainer had both been replaced within 10K miles prior to the surging problem. This doesn't rule out the sock being a problem or a contributing problem in your case, but it's context from my experience.

Also, I forgot to mention that, just before pulling onto the Interstate where surging occured, it first happened under load and acceleration at much lower speeds (25 - 35 mph) but in the same RPM window. So our trucks' symptoms seem nearly identical.
 
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For what it's worth, my in line fuel filter and fuel sock / strainer had both been replaced within 10K miles prior to the surging problem. This doesn't rule out the sock being a problem or a contributing problem in your case, but it's context from my experience.

Also, I forgot to mention that, just before pulling onto the Interstate where surging occured, it first happened under load and acceleration at much lower speeds (25 - 35 mph) but in the same RPM window. So our trucks' symptoms seem nearly identical.
Do you have an intact EGR system? I have desmogged, it's hard to say if this is a factor. This thread 3FE- Loss of power/mild stumbling/at mid RPM range (Fixed it Thread) Whew. seems sort of similar to the issue, and it was desmogged.

For this guy, it was a combo TPS/O2 sensor issue. Could an O2 sensor problem explain these symptoms? I've read rumors on the forums that after warmup the EFI system relies very heavily on O2 sensor input, which would explain my 20ish minute timeframe before the issue shows up.
 
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Do you have an intact EGR system? I have desmogged, it's hard to say if this is a factor. This thread 3FE- Loss of power/mild stumbling/at mid RPM range (Fixed it Thread) Whew. seems sort of similar to the issue, and it was desmogged.

For this guy, it was a combo TPS/O2 sensor issue. Could an O2 sensor problem explain these symptoms? I've read rumors on the forums that after warmup the EFI system relies very heavily on O2 sensor input, which would explain my 20ish minute timeframe before the issue shows up.

Yes, my EGR system is intact with the original EGR valve assembly (I've searched for years for a replacement but it's discontinued).

I have not replaced O2 sensor. Your 20 minute warm-up observation is a good one. It reminds me that, before any work happened with my truck, I took a few rides with real mechanics to try to replicate the surging. But none of those rides was for more than 10 or 15 minutes, and I couldn't get it to surge again. So, the same 20 minute timeframe may be at work in my case.

I'll read that thread that you linked to. I'm not at all convinced that I'm entirely out of the woods on this yet, so thanks for posting it.
 
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Do you have an intact EGR system? I have desmogged, it's hard to say if this is a factor. This thread 3FE- Loss of power/mild stumbling/at mid RPM range (Fixed it Thread) Whew. seems sort of similar to the issue, and it was desmogged.

For this guy, it was a combo TPS/O2 sensor issue. Could an O2 sensor problem explain these symptoms? I've read rumors on the forums that after warmup the EFI system relies very heavily on O2 sensor input, which would explain my 20ish minute timeframe before the issue shows up.

After reading that thread, I wonder if TPS and O2 sensor replacement would be good preventative maintenance regardless.
 
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This certainly sounds like fuel starvation to me.
I had very similar symptoms due to a clogged pre-filter (fuel sock) on the fuel pump. As the fuel in the tank warms, the pre-filter will collapse on itself and block the intake to the pump. This would usually happen after an hour of driving, but was dependent on ambient temps.

Another possibility would be over/under pressurization of the fuel tank due to a faulty charcoal canister, plugged vent line, or even a bad fuel fill cap.
I had this issue climbing I70 out of Denver on the way to Moab. I had a crushed vent line by the fuel filter that I didn't notice after my font end exploded earlier that year. I bypassed the hard line and all was good.
 
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This certainly sounds like fuel starvation to me.
I had very similar symptoms due to a clogged pre-filter (fuel sock) on the fuel pump. As the fuel in the tank warms, the pre-filter will collapse on itself and block the intake to the pump. This would usually happen after an hour of driving, but was dependent on ambient temps.

Another possibility would be over/under pressurization of the fuel tank due to a faulty charcoal canister, plugged vent line, or even a bad fuel fill cap.
I had this issue climbing I70 out of Denver on the way to Moab. I had a crushed vent line by the fuel filter that I didn't notice after my font end exploded earlier that year. I bypassed the hard line and all was good.
Thank you very much Jon. I did the fuel pump recently and put a new sock on and it didn't fix the issue.
Faulty charcoal canister, though, is one that I really want to fix and has been a known issue due to vacccum whooshing at the tank. Getting a replacement canister in Canada has been a nightmare, though, even with the help from posts on it. I've resolved, after reading your post, to finally just get something close and make it work.

Do you have any advice on where to find the vent lines? I wasn't able to spot that or the fuel filter location in the FSM but fortunately I already know where the filter is and have ordered it.

It's neat how this is a temperature issue. I had the problem start last year in August on my way to the Rockies. No issues in the winter at all, but once ambient temps got up past 20 C/70 F this year the problem returned and is worse.
 
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Do you have any advice on where to find the vent lines? I wasn't able to spot that or the fuel filter location in the FSM but fortunately I already know where the filter is and have ordered it.
The vent/return lines are all run along the passenger side frame rail and can be seen in the area of the fuel filter. The FPR fuel return line, the feed from the FP to the fuel filter, and vent line are all run together. They change from soft lines to hard lines a few times in the run.
The fuel filter is mounted to the inside of the right frame rail directly below the distributor. If you look at this post, you will see a blurry cell phone picture of the fuel filter.
 

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