97 stumbles/ hesitates on light acceleration and low/rough idle, but only in the summer??

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Finally bringing myself to ask for some help tracking down this issue with my 17 year old son's daily driver. 180k miles..... the motor stumbles/ hesitates on light acceleration and has a low/rough idle, but only in the summer?? It runs like a top when its cool / cold outside. Outdoor temps get in the 80s and the issues return.. but not always.... about half the time it runs fine. We have been chasing this problem for over a year now, and all of our efforts have failed to remedy the problem.

There is No CEL. When we first bought the truck it had a bank 1 lean code which went away after some of our efforts below. I think when we replaced the fuel pump.

Parts that my son and I have replaced over the last 12-18 months trying to fix the issue: fuel pump, injectors, fuel filter, ignition coil, distributor cap, adjusted timing, replaced pcv valve , replaced 02 sensors. fusible link + junction block,. checked spark plug wires, checked spark plug gaps. checked all vacuum lines. EGR delete was performed by a shop in March 2021 when i first acquired the vehicle. Otherwise we have done all the work ourselves.

I tried to lay it out to the best of my knowledge, but if there is any other info that would be helpful in diagnosing this issue please let me know and i will try to track it down. Any ideas or advice or just a point in the direction of a resource would be massively appreciated. I'm running out of ideas!
 
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How was the egr delete done? What o2 sensors used? Is vehicle always in open loop?
 
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How was the egr delete done? What o2 sensors used? Is vehicle always in open loop?
EGR delete was done with a wits end kit. i believe the o2 sensors were napa. I'm not sure what you mean by open loop?
 
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Have you replaced the engine temperature sensor? There are two, one is for the dash gauge the other supplies data to the ECM.
have not tried engine temperature sensor. the temp on the dash is reading normal. i will see if i can look into these more. thank you. FWIW, i dont' understand why it would only be doing this in the hotter summer months. possibly a coincidence, but odd.
 
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have not tried engine temperature sensor. the temp on the dash is reading normal. i will see if i can look into these more. thank you. FWIW, i dont' understand why it would only be doing this in the hotter summer months. possibly a coincidence, but odd.
There are two temp sensors.

One is for the gauge on the dash. The other is to tell the ECU what to do.

Once it reaches a certain temperature, it tells the fuel pump and O2 sensors to go to closed loop and decrease the fuel pump speed and monitor the O2 sensors more.

If you are in open loop, the fuel pump runs faster to allow more fuel because it thinks it's "cold", so just like having the choke on in cold weather. Once the engine warms up it goes to closed loop and it's like the engine is warmed up and the choke is not needed anymore.

However, if you have one condition at the opposite of when you want it, it won't run well.

I don't recall which sensor is which, but I know there have been a few who have posted that info here.
 
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There are two temp sensors.

One is for the gauge on the dash. The other is to tell the ECU what to do.

Once it reaches a certain temperature, it tells the fuel pump and O2 sensors to go to closed loop and decrease the fuel pump speed and monitor the O2 sensors more.

If you are in open loop, the fuel pump runs faster to allow more fuel because it thinks it's "cold", so just like having the choke on in cold weather. Once the engine warms up it goes to closed loop and it's like the engine is warmed up and the choke is not needed anymore.

However, if you have one condition at the opposite of when you want it, it won't run well.

I don't recall which sensor is which, but I know there have been a few who have posted that info here.
this is super helpful. i will dig into this more, replace as temp sensors as needed and report back. huge thanks for the info.
 

jbee

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Check this thread and search google for FZJ80 temp sensor.

 

John Young

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I had similar issues in my 93. Nobody could figure it out. Turned out to be bad wiring on the air flow sensor. I'd suggest looking at all the engine-related wiring and poke at anything suspect. Good luck!
 
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thanks for the links and comments. for the sensor, i believe we are talking about (c) in the diagram below?


1660762164877.png
 

John Young

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thanks for the links and comments. for the sensor, i believe we are talking about (c) in the diagram below?


View attachment 3089380
They sure make it look easy to reach in there and get at those things. It was not the case for me. I agree, BTW, that just replacing sensors in general is a great idea. They are 25 years old at this point and at the least the plastic on the connectors is brittle as hell.
 
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temp sensor was pretty gross and brittle, but changing it out had no impact to the crummy idle, etc. still glad i changed it. i took a closer look at the EGR delete, i said wits end, but that was just the "key chain" they sell. the plug looks similar to the one sold by mud vendor (pic below) ? didn't see anything visibly wrong with any wiring around the MAF but the main wiring harness near where the EGR would have been has been hot, and main insulation is exposing some of the wires (no bare wires that i can see) which you can also see from the pic... the truck is idling at only about 250 with AC off, which i know is way low, possibly same culprit?
EGR test.jpg
 
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Check for air leaks on the air intake hose. Sometimes they are cracked but you can’t tell until you remove it any inspect it. Any kind of leak will bugger things up.
 

John Young

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no need to guess. get a leak test smoke machine on amazon and put it in the intake, you'll find the leaks fast.

my truck idles so smooth now sometimes I think it turned off at red lights :cool:
Could you expand on that a little, HA? I thought the smoke machine was mostly to look for vacuum leaks--you see where the smoke wants to go. How does smoke on the intake work? Sorry for my ignorance. I've never used one but thought about buying one from time to time. I have a couple of rough-running carbureted 1FZ trucks
 
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Like a couple others mentioned, check the intake tube after the air cleaner for cracks. This is easy to check and easy to replace.

You said the harness by the egr has seen some heat and it's showing wires. Definitely open this up and investigate. Make sure there is no metal wire exposed and get the harness repaired and wrapped up well.

Also make sure there are no vacuum leaks.

If all that checks out and doesn't fix the issue than I would recommend doing some research and gaining a thorough knowledge of a proper egr delete and than make sure it was done proper.

Goodluck
 

haulin auss

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Could you expand on that a little, HA? I thought the smoke machine was mostly to look for vacuum leaks--you see where the smoke wants to go. How does smoke on the intake work? Sorry for my ignorance. I've never used one but thought about buying one from time to time. I have a couple of rough-running carbureted 1FZ trucks

It's pretty straightforward, you hook the smoke up to the intake to test for leaks in the intake plumbing and vacuum hoses.
If you're interested in exhaust leaks, you can then hook it up to the exhaust to find those.
 

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