Hello, I've lurked for a few months and finally jumped into a really clean '99 4Runner a few weeks ago. Love it so far.

However it recently started an intermittent issue: it will stumble when accelerating, but only at low highway speeds. For example accelerating to pass someone at 50 -> 60 mph. Around town it accelerates smoothly at all RPMs and no check engine light it on. It isn't predictable; sometimes it happens and sometimes it doesn't.

Potentially related issue: hot starts are a little rough. Always starts, but not as sharply as cold starts.

I'm no stranger to DIY work and and EFI systems, any test procedures are appreciated! My first impression is MAF, since it happens when I expect the manifold pressure to approach atmospheric. Or it could be bad gasoline, time will tell. New plugs, wires, fuel filter, and air filter are on the way. New oil and filter this past weekend but I don't see how that could be related.

 

Check really good for vacuum leaks. Check to make sure your fuel pressure regulator is hooked up. Check for collapsed vacuum lines. Plugs and wires could very well be the culprit too. Have you pulled a plug to inspect it's gap/color? Maf is rock solid, as are the coil packs, TPS, and ECU.

 

When this occurs, do the RPMs jump back and forth or does the throttle feel like its stuck? This might be an issued with your TPS.

 

Thanks for the replies.

Re: spark plugs. Haven't pulled one yet, am planning to do them shortly when parts come in. I thought there was an error when the total was $26 just for the plugs... but I guess that's just how much they are for the correct NGK ones.

Re: TPS. RPMs don't jump/fall. Wouldn't that indicate a faulty crank sensor if they did? I did some transitions from idle to partial throttle, and then to WOT around town and it's smooth. I'm assuming these are potentiometer TPS units, is that correct? So a worn area in the track would affect the reading at that TPS % regardless of load.

 

Clean your mass airflow sensor. It could have dirt or a bug on one of the sensors.
It has happened to me.
Use SENSOR cleaner only.

 

Issue solved and wanted to share what the problem was in case anyone has similar issues in the future.

I pulled the old plugs and the plug wires. Plugs were definitely due for replacement, but looked even across all cylinders. I'm wondering if they're factory plugs... it had the Denso plugs on the passenger bank and NGK on the driver side bank.

The coil wires were the problem though, I believe. Cylinder 4 boot was trash. I believe the misfire was caused by the spark arcing to ground through this boot, leading to its current appearance.

All parts replaced with the correct OEM parts, hot start issue went away immediately and pulls harder.

 

I had stock Denso and NGK plugs in my Tacoma when I bought it at 250k miles. It wouldn't accelerate either...