Odd engine stumble.

[MikeDiehl]

Hi all,

About a month or so ago, my son and I replaced the valve cover gaskets on our '96 and now the engine stumbles at 1200-1300 RPM.

When we did the work, we were trying to resolve a check engine light that indicated a #5 misfire. Inspection showed oil leaking onto the spark plugs. As part of the work, we replaced the gaskets, grommets, and the seals around the spark plug tubes. We torqued everything to spec. We are pretty sure we didn't roll the spark plug seas, or leave gaps in the gaskets. We did both cylinder banks. Finally, we changed the spark plugs and added a fuel cleaner.

So, after driving the truck a bit, we see that it (often, but not always) stumbles at around 1300 RPM. This speed seems to change to about 1500 RPM when we have ECT enabled.

Any thoughts on what this could be?

Thanks in advance,
Mike Diehl
'96 with 260K.

 

[eternalize]

Could you give us more information to what you mean by "stumble"? I know you said misfire, but how do the RPMs read around that range when you say it stumbles?

 

[MikeDiehl]

When I say "stumble" I'm referring to driving along with the engine running smoothly and then all of a sudden, we see a drastic reduction in power and the engine isn't running smoothly. This lasts for about a second or 2 and then things are back to normal. Since this is usually when we are under acceleration, the rpm would be rising through this point on the tach. The reference to a misfire was simply meant as history of recent mechanical work that we did on the engine.

I hope that helps.

Mike.

 

[Ben Dejo]

Recheck the electrical connections that you may have disconnected to get the valve covers off, vacuum hoses/evap/egr lines, and grounds. Could be cam position sensors not connected well, could be a vacuum leak on old cracking hoses. ECT changes the shift point to a higher RPM, so that appears to be working normally. There are lots of connections to keep track of. Listen for new noises or noises that changed. I forgot to reconnect the PCV but that really made the engine stumble and obvious vacuum noise. My point is something up front and in view can be easily missed. Good luck.

 

[T4R2014]

Inspect coil boots, move coils around. If the wires are older than 3 years and not OEMs, I would replace.

 

[Zhehan]

Replace the coil boots as well. They should be replaced when you replace wires.

 

[MikeDiehl]

Follow up

Well, the truck has been down for a while so we could repair a problem with the door latch. We checked for loose wires and hoses and only found a loose bolt holding one of the other ignition coils down. Nothing else.

We are considering the advise to change the ignition coils/wires/boots. Is there any way to test the coils except to swap them around?

Otherwise, my son was playing with chat gpt and got this:

Yes — an exhaust manifold leak at Cylinder 1 can absolutely cause a stumble right around 1500–1800 rpm.
That RPM range is where the 5VZ-FE is especially sensitive to mixture problems caused by a leak.

Here’s why it happens:

Why an exhaust leak can cause a stumble at ~1700 rpm
At light acceleration around 1500–2000 rpm, the ECU is switching between:
Closed-loop (O2 sensor-controlled fuel)
Open-loop (map-based fuel)
If there’s an exhaust leak at Cylinder 1:

✔ The O2 sensor sees false extra oxygen

→ It thinks the engine is lean
→ ECU adds fuel
→ Bank 1 gets too rich

At certain airflow levels (especially ~1700 rpm), that extra fuel causes:

• A stumble / hesitation
• Slight bog
• Minor misfire feeling

This is extremely common with:
A blown gasket at Cylinder 1
A cracked manifold
A loose or broken stud

Why the stumble happens at 1700 specifically

The 5VZ-FE has a known “transition zone” around 1600–1800 rpm where:

The O2 sensor is very active
The ECU is trimming fuel
Small mixture errors are exaggerated
Exhaust leaks create the biggest AFR swings

So a leak on Cylinder 1 can create a stumble at that exact RPM even if it’s smooth at idle or higher RPM.

Signs your stumble IS from a manifold leak
Check if these match your truck:

✔ Ticking noise on cold start (fades when warm)
✔ Cylinder 1 misfire or occasional P0301
✔ Slight exhaust smell near passenger wheel
✔ Stumble only under light throttle around 1500–2000 rpm
✔ Smoother when fully warmed up
✔ Stumble decreases when accelerating harder

If you matched 2–3 of these, the manifold leak is the number one suspect.

Is chat gpt smoking crack (again?) or is this plausible?

TIA,
Mike

 

[Ben Dejo]

Hi Mike,
I haven't begun to trust ChatGPT yet.
The last two comments from T4R2014 and Zhehan are great advice. New spark plug wires and new ignition boots for the coils is a less expensive place to start. I wouldn't replace the coils just yet, since they are like $100 each for the Denso replacements. I wasted money doing the parts cannon thing replacing the coils....and so many other parts that didn't need to be replaced.
You can test the resistance of each of the coils if you have a multimeter. I did A quick search on google and I found this post with the correct resistance measurements from the FSM or the Haynes manual (If you don't have a Haynes manual, I suggest getting one. you can find one at Autozone or any of the parts stores). Need a Little Help, Can Someone Measure Secondary Coil Resistance | Tacoma World

Did you resolve the misfire on #5?. I want to say OBD2 is on all the 3rd gen 4runners, but I could be wrong. Do you have your own OBD2 code reader? You could track the short term fuel trim and long term fuel trims to see if they are +/-20. There are tons of threads regarding that on the 3rd gen fourm. Anyway, I was thinking about a failing injector on #5 that could be an issue, which sometimes causes a stumble (kinda more than a stumble) after driving somewhere, turning off the engine (like going to a store) then the stumble/misfire shows up when you start/drive again, once the engine/injectors heat soak a bit. You have different injectors than I do, as I have a 99. Hopefully others that have a bit older 3rd gen than me could chime in here if the injector issue is different or the same with theirs.

 

[BuffaloBass]

WHAT spark plugs did you replace with> ? Dual strap as oem ? which if not could cause issue.

PS I always check wire resistance when I play with them to ensure non are broken. ( which is easy to do )

 

[T4R2014]

There's an FSM method to test resistance on the coils. Mine tested within spec, but new Denso coils made it feel like I gained power. Probably better to test them with an oscilloscope, but they are cheap enough so why bother?

Even single electrode plugs are unlikely to cause this. To me this sounds like wires though. If not, I'd be looking at the ignition module.

But we also don't know if this is actually an ignition issue yet.