Misfire 3K+ Hard Throttle

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xveganxcowboyx

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I have been working my way through possible causes of a misfire in my 99 4runner. The truck runs great at low RPM and partial throttle. Around 3,000 RPM and above I get a consistent stumble. If the throttle is past 30-40% it becomes a legitimate misfire, very serious power interruption and a flashing CEL. Codes in the past have shown Lean Bank 1, Misfire (cylinders 1, 3, and 6 at different times if I remember correctly). MPG has also been down, but that could be due to the weather.

I have done:
Rebuilt supercharger
Rebuilt injectors
New MAF
New O2 sensor
New plugs (1 step colder than stock)
New plug wires
New fuel filter
Compression is between 160 and 190 cold
Fuel is BP 93 octane

I just tested the TPS. It reads smooth from 0.54 volts to about 3.8. I feel like this should be 4.5 or better to register WOT.

I am thinking I should check fuel pressure and maybe check the coils (but it doesn't seem like a coil issue.)
 
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I would tend to think that, but it happens at fairly low RPM (3,000) and lowish load. The truck has had the supercharger on it for most of it's 249k life and only recently begun developing this issue.

I just checked my codes, the last "incident" yielded a misfire in cylinder 1 and no other codes.
 
I know you said you rebuilt your injectors but sounds injector related. The cylinder 1 injector is easy to get to maybe switch injectors around on that side and see if the misfire changes cylinders.
 
The injectors have all been changed for a rebuilt set. The issue was the same before and after the injector change. They are tough to get to with the supercharger on so I changed them all as a precaution when I had it off.


*Update* I tested my fuel pressure and found it is a steady 48psi with and without the vacuum hose attached. It rises with the addition of pressure though. I'll have to look up the specs, but I would have expected 43.5 without the hose and ~35 with it attached. I don't know much about the fuel pressure regulators used with these superchargers though.
 
GrampsT4R said:
Is it the vtec kicking in?

Sorry, had to.
Click to expand...


Not on this one. I had a car with VTEC, but it kicked in at 6,000RPM. The fun range on that car was 7,000-9,000RPM. I'm pretty sure those numbers would eat valves and break cylinder walls on my 4runner. The 4runner makes gobs more torque down low though. :boxer:
 
Any new ideas?

I'm going to try swapping coil packs when I get home.

I am also leaning toward changing the fuel pump. Just because it is making good pressure at idle doesn't necessarily mean it is keeping up under load. I have only ever had fuel pumps work, fail, or stick. I've heard of fuel pumps getting weak or socks getting clogged, but does anyone have any real experience with that? Also, how tough is it to drop the tank down to replace the fuel pump? I have a few pumps sitting around from DSMs which should provide plenty of flow and still be reliable and quiet.
 
So far the coil pack (#1) change seems to have cured the problem, but I am going to remain skeptical, because it felt the same way after I changed the wires. I'm still curious as to why I had misfires in multiple cylinders and why a lean condition was reported if it was an ignition issue. I have ~400 miles to put on this weekend for a trip so I'll see how it's doing after all that.

I decided to apply some belt dressing to the supercharger belt as well. It likes to slip at high RPM in the cold. Now it pulls very nicely all the way through the rev range. :lookout:
 
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