HelpMeFixIt!!

I know this has been talked about in the past, I've searched many times on it.
I wanted to combine my info in my spot in the hopes of gathering some much needed support

I started a thread on another 4 Runner forum, but not much feedback so hoping this forum will yield more experienced responses.

Here's the scoop:

Previous owner was driving on highway and 60+ mph when it started stumbling at higher rpms. He was low on gas so filled it up and barely made it home. With the full tank, condition got worse. After sitting for approx 2 weeks, it started doing what its continuing to do now, running very rough at about 3500rpm with max rpm of about 4k. Conditions worsens the more gas you give it and it cant do any better than 4k, running very very bad at that rpm.

We've done the following:

Swapped ECU
Removed CAT - also dropped exhaust to make sure muffler wasnt the issue
Fuel pump replaced
Fuel filter replaced
TPS tested with meter - its good
Plugs, coil packs and wires replaced (ngk plugs)
Cam AND Crank Sensor replaced
Fuel pressure tested, no issues
Vacuum pressure tested - vacuum was smooth at 1000rpm increments, no fluctuation at all. At about 3600rpm the pressure started to drop, but still no fluctuations. Full throttle eventually brought the vacuum to zero. Even with the terrible condition that occurs at 3600, vacuum was steady at about 15.
MAF tested with meter and cleaned with MAF cleaner
Throttle body cleaned and seafoam engine cleaner as well
Timing belt, water pump and pulleys replaced. Timing is correct, cam marks, crank marks all line up as they should

It behaves in this manner whether its in park, neutral or if you're driving it in D, L or 2. It never changes, but pulls and runs normal up to 3500.

Hopefully I didnt leave anything out.

Oh....before anyone ask the obvious - NO CODES.....EVER.....
We can force a code by pulling the maf, tps etc, but no codes are generated with everything in place. CEL blinks at worst point of the issue, over 3600, but no codes are left.

We were thinking weak valve spring, valve float, but vacuum test doesnt indicate any issues at all.

Previous owner says the MAF was swapped with a brand new one from NAPA when this problem started and nothing changed so the original was put back in. We've tested it with a meter and it appears to be good.

Ignitor?? Dont know how to test this - under the impression it either works or it doesnt or causes issues once hot. That doesnt fit our scenario.

Any info/suggestions at all will be HIGHLY appreciated.

Scott

- Adding that the injectors were sent to witchunter.com for professional cleaning and testing

Sirotto

Go do an ethanol test on the fuel. Once a vehicle gets to much ethanol it will be a time bomb till something plugs the injectors. Sound like the same issue i had on another SUV. The first batch will pull some rubber from the fuel lines and impact it against the backside of the pintle. A second batch will cause more issues.

Ethanol test and pull the injectors, its probably leaning out as the RPM come up till it is starved. Enough times and it should set off the knock sensor though.

JasonYota

iTrader: (1)

Have you checked for "pending" codes. The malfunction has to happen x amount of times before it triggers the CEL.

HelpMeFixIt!!

Updated my post - we did send the injectors off to witchunter.com to get professionally cleaned and tested - all's good there

HelpMeFixIt!!

Its been scanned by the dealership and by us many times.
This issue has occurred countless times over the past year.

The previous owner is a friend and the truck started to have this issue over a year ago. Its been to the dealer, his mechanic friend and now us.
We got the truck for $500 so figured it couldn't hurt to try to work this out.

JasonYota

iTrader: (1)

If the CEL blinks when the problem is happening then it's setting a code. Have you drove it with a live data scanner to see what the computer is doing?

HelpMeFixIt!!

I'm curious where that code would be stored then....

And unfortunately no, we have no such live data scanner.
Will have to look into that and find someone that does.

Thanks....

HelpMeFixIt!!

Dad took it to a mechanic that ran a live data scanner. Nothing found so far.
They were able to swap the maf with a known good one from a used car lot, so now we know for sure maf is good. They're still investigating.
Knock sensors came up in conversation.

Just_Me69

Is the crank pulley for the t-belt on all the way? Mine did the same thing when my crank bolt was loose. The crank gear and pulley walked out until the reluctor wasn't close enough to the crank sensor.

rworegon

Check the intake tube and all other tubes and gaskets for a vacuum leak. PCV and grommet replaced too? Could a lazy O2 sensor be the culprit? I'm surprised something did not show up on the live stream data.

HelpMeFixIt!!

It's been removed and put back when the TB was replaced.

HelpMeFixIt!!

I'll know more tomorrow afternoon. They still have the truck and testing.

Jomoka

You can unplug them completely and it won't run like that.

Actually, that might be something to try, who knows. Unplug them and see if it helps.

HelpMeFixIt!!

Will do..Cant hurt..

HelpMeFixIt!!

UPDATE -

Crank sensors tested and wiring harness replaced
Test performed with wiring harness unplugged as well.

Igniter tested to the best of our ability, but there was no code, so testing didnt really make sense.
Ordered a replacement since it was only $20 from ebay.

Manually checked timing
Took cam pulley cover off - manually turned motor over till both cam pulleys lined up TDC.
Had dowel rod in spark plug tunnel to verify visually that piston 1 was actually at TDC.
Crank pulley was at the zero degree mark. All is good here.

If the igniter doesnt do it, we're pretty much done. Everything else has been tested or placed - vacuum lines and pressure, fuel pressure etc.
I think we've replaced every part but the motor itself at this point.

I still cant get over how perfectly this thing idles and runs up to that 3500 mark.

Oh...and fuel pressure regulator was replaced as well...

Expert that visited thought exhaust like others have said with Cat etc, but dropping the entire exhaust at the pre cat connection did nothing.

We're seriously lost on this now....

HelpMeFixIt!!

Bttt..

donomite49

A puzzler for sure, I agree with some others that it must be exhaust related, since it throws no codes and so many things have been checked or replaced.

When you say it stumbles, describe exactly what it does like misses or falls flat on accelleration or what.

A clogged cat or muffler with a hole in it will make it run flat, especially on top end.

HelpMeFixIt!!

It literally stumbles, like hitting the rev limiter at first but if you continue to give it throttle the stumbling becomes more erratic and worse.
I've seen it rev to about 4k or so as you give it throttle but its running so poorly at that point it sounds like its about to explode.

Up to 3500 though, purs like a kitten.....

And is there anything in front of the cat under the truck? Doesnt this model only have one cat? The cat has been removed and the truck run with open exhaust from where the cat was and the problem didnt change at all. So unless theres something else between the head and the cat, I dont think its exhaust.

TheDurk

Yeah, 1998 should be only one cat, and it's just pipe/manifold going upstream from there. You can confirm by looking at top left corner of emissions label under hood. One-cat system will say SFI, TWC, HO2S(2). Two cats will say SFI, A/F, TWC(2), HO2S. Or similar.

vital22re

iTrader: (2)

IF the check engine light blinks, there's a code. If the ecu isn't storing it, then the ecu may not have power. There is a 12v constant to the ecu and a 12v on with ignition. Verify that the ecu has both at the correct spots. IF you hook up a live data capable scanner, are all the monitors run and complete? evap, 02's, cat, If they are not complete, then the ecu is reseting every time you shut off the truck.