Engin Hesitation - Complicated by a SC

Loki94

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Hello all,

I'm at my Wit's End ;) with engine hesitation and pinging (i.e running lean) complicated by having a supercharger installed by the previous owner (who owned a Toyota dealership).

It has:
  • Walbro 255 fuel pump
  • TRD Fuel Pressure regulator
  • I always run high octane gas
The truck ran great at the time I purchased it. However, the original downpipe rusted and developed the expected code 28 and ran terribly.

I'm working from home due to COVID, so I thought what a great opportunity to learn new welding skills and install a 3-inch over-frame exhaust.
  • Due to the style of the downpipe, I had to weld bung type fittings. I did a ton of research on the proper O2 sensors (this is highly debated) but I settled on Denso 234-9042 Air Fuel Ratio Sensors (I think they are as closest to OEM I can get)
  • I also installed new plugs ( wires, distributor cap, rotor, Denso Spark Plug Kit for Boosted 1FZs & FZJ80s) actually from wit's end :p
  • New K&N filter
  • New OEM fuel filter
It ran marginally better, code 28 didn’t return, but still cutting out on hills and any attempt at acceleration. Has the expected pinging with running lean, but it is terrible. The battery was disconnected for a long time, so I thought it needed to relearn especially given all the new going on. So, I babied it at 2K RPM for the last 2 weeks and shifted to lower gears to be able to reasonably get up hills without cutting out.

Well, this doesn’t seem to be getting any better with time, so here I am.
  • No codes (other than ABS code 35 (front left or rear right sensor)? I don’t think its relevant to this, but I cant explain because it wasn’t present before I started this endeavor.
  • I can’t find any issues with the vacuum lines
  • If I disconnect the vacuum line from the fuel pressure regulator it runs better. This is for diagnostic purposes, I will not leave it that way.
  • The new headers eliminate the ERG port, so I thought it might be the EGR temp sensor (I did the 1 ohm resistor bypass and BB EGR delete). This actually made things worse.
  • To rule out bogging down due to shifting issues, I made sure the transmission fluid was adequate.
  • There is a lot of discussion about a 7th injector and I might do that too someday, but like I mentioned it was running fine before all this work.
Any suggestions about what to do next? The last thing I can think of is a new fuel pump – which I have not done yet, but I thought I would get some other opinions before doing that or just taking it to some professionals.

:bang:
 

S.CarolinaFZJ80

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Lose the k & n. The oil messes up the MAF sensor. Clean up the MAF sensor, ohm it out per the FSM. Get a new one if it doesn't pass the parameters.

And check the timing
 
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If it’s honestly detonating, it should be throwing a code too. But pull your plugs and check them. You probably need colder plugs. I ended up 2 heat ranges colder on mine. BRK7E NGK IIRC.
 

ppc

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I would start by opening up the wiring harness that passes by the EGR and look for damaged wires. Even if the outside looks OK the wires can still be damaged. After that I would make sure the in tank fuel pump has the filter sock properly attached, the wiring to the pump is OK in the tank as well as the harness/connectors going to the tank. I would also verify the fuel pressure at the fuel rail under driving conditions.
 
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Yeah if you don’t have a 7th injector, meth injection and/or Intercooler you are going to have problems. There is a thread by @cruiserdan and @Photoman that goes into a lot of detail about this.
Intercooler without added fuel will just make it worse. He is going to have to add fuel with an injector or large methanol injection.

Put a wideband A/F gauge in your new downpipe and you will see you are going lean. This is exacerbated by adding a larger exhaust.
 

Squash

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Carefully open up and remove the air intake sensor. Use a sensor cleaner or brake clean and let it dry before putting her back together.
As previously mentioned the K&N will pollute.
X2 wiring harness. How does it behave in reverse? Bad wiring harness seems to always be worse going forward.
 

ppc

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Carefully open up and remove the air intake sensor. Use a sensor cleaner or brake clean and let it dry before putting her back together.
As previously mentioned the K&N will pollute.
X2 wiring harness. How does it behave in reverse? Bad wiring harness seems to always be worse going forward.

Do not open up the the VAF found in the 94 model year. The air path would not have any sensor/electronics that would get contaminated by the oil used with the K&N filter. Opening or disconnecting the VAF by removing the screws on the electrical connector may destroy it. There is a metal clip that holds the connector in place. That can be removed to allow the for removal and testing of the device per FSM. Chances of the VAF being the problem with your symptoms is going to be very rare.
 

Loki94

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Do not open up the the VAF found in the 94 model year. The air path would not have any sensor/electronics that would get contaminated by the oil used with the K&N filter. Opening or disconnecting the VAF by removing the screws on the electrical connector may destroy it. There is a metal clip that holds the connector in place. That can be removed to allow the for removal and testing of the device per FSM. Chances of the VAF being the problem with your symptoms is going to be very rare.
Thank you for the info. I will get rid of the K&N, but will not open it up.
 

Loki94

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Carefully open up and remove the air intake sensor. Use a sensor cleaner or brake clean and let it dry before putting her back together.
As previously mentioned the K&N will pollute.
X2 wiring harness. How does it behave in reverse? Bad wiring harness seems to always be worse going forward.
Reverse is no issue, neither is 2nd (unless puling a long hill or low. Highway speeds are also not an issue as long as I take a long time getting there. So going above 60 at 35K or 45K RPM in low is not a problem.

I should have put this in my intro - At just above 45K RPM there is a noticeable bump in HP - I'm assuming it's getting the fuel it finally needs.
 
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I would take a look at the TPS and IAC wiring harnesses extensions required for blower install (unless they were unloomed and re routed)

Second would be transmission shift solenoids.

Both of these were the causes of similar issues for me (1995) and neither threw a code.


This part seems like perhaps a valuable clue.
Quote: If I disconnect the vacuum line from the fuel pressure regulator it runs better.
 
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Being a 94 to get rid of the egr, you run a jumper wire not resister in the plug. You must also do something with the vacuum lines for the egr and should block off both ends. Why are you trying to get rid of the egr? Have you tried restoring it to see if it gets better? O2 sensors for 93 94 are not denso they are NtK brand which you could get from rock auto.
 

Pierce

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TRD Fuel pressure regulator? I haven’t read of anyone using one on a SC 80 series. Didn’t know they existed.

I hate to throw more speculation at this but I had a 3rd Gen 4Runner with hesitation, backfiring and stalling, especially under and type of acceleration. After throwing all sorts of parts at it it turned out to be the Fuel Pressure regulator. Might be an inexpensive, easy to install start?
 

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