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Mambeau / Admin
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No wonder it was running badly ... A double whammy of jumped timing and two bad injectors ... :runaway:
 

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Discussion Starter #22 (Edited)
FML!!! Here we go again with this truck. UPDATE: Had the truck on hiatus for a while and just got it back on the road again.

Losing power on the bottom end (feels like a 2-stroke motor where power comes in in high RPM), shift to higher gear and it bogs down under any load. Threw CEL codes P0130 for O2 sensor (upstream bank 1 sensor 1) and P0510 for TPS; both replaced, no improvement on performance but new code: P0120 Throttle Pedal Position Sensor (Stealership parts guy says that this doesn't exist on 96 Taco 3.4L)

Heard rattling in exhaust, removed rear cat-back system, shook out several chunks of ceramic pieces (golf ball + size)....remember that glowing cat picture???

Fuel Economy went completely to hell (160 miles on full tank...ran out of gas last week) and it intermittently craps out on power and can hardly maintain fwy speed sometimes. Cannot find vacuum leaks. Plugs all look good but exhaust smells heavily of raw fuel.

Thoughts?
 

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Mambeau / Admin
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There is a code P0120 for the 1996 5VZ, and it can be caused by:

- A short or lost connection in the the TPS circuit
- The throttle position sensor itself
- The ECM

The code P0510 you previously got was thrown because the closed throttle position switch never activated. This could have been caused by an open / broken connection in the TPS circuit, a throttle position switch stuck in the closed position, or an ECM glitch.

My first guess is that it's an electrical fault in the TPS circuit / wiring.
 

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Discussion Starter #24
Why does that sound like it's going to be an S-ton of $$$$ to get that fixed?
 

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Discussion Starter #26
I also forgot to mention that I cleaned the MAF with an appropriate MAF cleaner; removed/cleaned the throttle body; removed and cleaned a gunky IAC valve. That seemed to clear up the rough idle but it idles at 18,00-2,000 rpm now.

All below info from OBDII scanner while at idle:

Saw Long term fuel trims at =14.8%

Short-Term Fuel trims were both at =19.5%

Ignition Advance was at 31 deg.

IAT Temp 40 deg C

CALC Load 16.1%

Coolant temp at 88 deg C

MAF (gr/S) 10.95

TPS 100%
 

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I see that you mentioned that you have changed the timing belt already. However I had a very similar sounding problem. And as it turned out even though my timing belt at the time was only 15K miles old it had slipped a cog. apparently the tensioner had not done its job. I suspect that the tensioner might not have been replaced. but everything you are reporting sounds like what my 95.5 tacoma was doing.
 

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Timing belt tensioner. Same thing happened after my rebuild, but I only made it 2500 miles before the 2stroke performance and wacky codes popped up. Brand new tensioner failed and the timing skipped.
 

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Discussion Starter #29
Took it to an independent shop that supposedly specialize in Toyotas the other day; the TPS wiring harness had the bottom pin pulled out just enough to cause a fault; that P0510 or 0120 have not returned.

Got rid of those codes and traded for an intermittent flashing CEL (when idling at stop lights) but then is steady. Now I have Misfire codes for Cyls 2, 4 &6 (Driver's side bank). Yes, this is proof that I am in a living purgatory.

The shop guy tried to sell me on plugs and wires for $300ish; I replaced the plugs and wires myself; the fuel rails were slightly leaking at the two banjo bolts so I replaced the gasket/metal o-rings to solve that problem.

Back to the shop again for further diagnosis (on their dime this time), they ran a compression test, clys 1, 3 &5 were about 175-180 psi and 2, 4 & 6 were 110-120psi. checked timing and it's right on. Did Seafoam spray in case valves were carboned up and not sealing correctly. Engine was completely rebuilt 15K ago so I'm having a hard time believing that all rings on only one side crapped out.

Still getting intermittent loss of power and CEL flashing at stoplights. I still am at a loss. should I just pull the head?
 

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I'd put money on your cam shaft for the 246 side being on the wrong tooth. Your timing belt needs to be redone! When your crank is on time that cam has the valves in the wrong position for the compression to be right...and for all that misfiring to happen.

Beating the same problem up with the same attempted repairs that keep failing will never solve your problem.
 

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Discussion Starter #31
Checked timing and it's dead-on. Fuel Trims are high so I adjusted the idle screw slightly; now it idles cold between 1200-1500 RPM and about 750-850 RPM at temp.

Still getting misfire codes on 2,4, & 6. Switched to DENSO OEM plugs, and swapped out MAF for one that I know is working properly.

I put Uderdog's Rear O2 sensor delete on since I had to punch out the damaged Cat Converter.

Still getting sluggish acceleration like something in the exhaust is choking itself out, but then a burst of acceleration. Things really bog down in low RPM range.

Still at a loss for next steps.
 

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Discussion Starter #32
Still having issues; when cold (after start-up) the truck has very little/no power in any form of acceleration and is very jerky.

Once it warms up, things tend to smooth out once you get into upper RPMs; any load like driving up an incline requires down-shifting and revving like a mother****er...lol.

Talked to Gadget and told him about misfires on 2, 4 & 6 with each cyls being relatively uniformly low on compression.

Enola,
Could the source be the scissor gear in the head on the Cams? If the teeth are off a bit (where the dimpled marks on the gears like up) COULD that cause all cyls on that bank to be low on compression?

My Diagnostic Dice are INOP right now...lol.

Everything else timing-wise lines up.
 

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It really sounds like a cam timing issue.
Have you ever taken the cam sprocket off when resetting the belt? I have seen some shear the cam key off so that the sprocket is advanced from where the cams are actually at. You won't see this until the sprocket is actually removed. Looking at the sprocket bolt from in front of the motor everything looks normal. This happens when a cam binds up..usually from lubrication issues
You say the motor was worked on before ... So..I did have one that sheared the cam dowel pin and someone installed another cam that looked the same..but when installed with marks lined up..had low compression on that bank..that was a head scratcherrrr.. Really not sure what that cam was for..possibly the opposite bank or a 3vz-fe? Put in another cam and ran fine
 

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Discussion Starter #34
Cam pins are all good, actually replaced with new ones when motor was rebuilt. Recently checked and all is well.
 

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Hey Brian.d.bay did you ever solve your issue with your yota? i'm experiencing similar things after installing a brand new longblock and tons of other new parts. its now overheating and getting random misfires. radiator is hot on top and cold as hell on bottom. pulled and checked thermostat. it opens at 180* with jiggle down. i took it out. now i get flow thru the system but running at a very cold 140*F. laser heat scanner tells me cyl 2 header is 300 degrees and all others are 500. sparkplug on 2 is black. cel is saying misfire on 3. ive changed fuel injectors, sparkplugs with dual electrodes, wires, checked coils, ECT since it was broke and temp sender too. any ideas guys? im stumped. i got lots more info too that ive checked and replaced including fuel pressure reg and pump.
 

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Mambeau / Admin
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(1) Welcome to CT ...

(2) Clues, please ...

  • What are the details (year, model, type, etc.) on both the engine and the vehicle?
  • Is the block the only engine portion that's been replaced (that isn't original to the vehicle)?
  • What specific CEL's are you getting?
  • Is there any reason why one should suspect a problem with the head(s)?
  • Is there any chance the wires were crossed between cylinder 2 and 3?
 

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hello thanks for the reply. it is a 1997 tacoma 3.4l manual 4x4 288k miles.
I ran it low on gas since I forgot my wallet. after that day it was misfiring on cyl 3. around that time the truck started using large amounts of coolant after it overheated bad one day. I ordered a fully refurbished long block with heads. I replaced the engine myself and all the timing components, bought refurbished denso fuel injectors, sparkplug n wires, radiator.... when I started it for the first time it barely ran. shook bad. got cels. p0300, 302 and 305. i instantly thought coil since both cylinders run on that coil. replaced it. nothing changed. i realized that since i ran it low on gas it was misfiring so i figured it was the fuel system. I replaced the fuel pressure regulator. it helped alot butstill misfired. it ran good enough to actually let it warm up. that's when i realized i had a overheating problem.
I've replaced top and bottom radiator hoses, radiator, thermostat, ect sensor and sender. top of radiator is hot, bottom is cold. it will start up and run decent until temp starts to climb, then it starts to misfire more prominently. i made absolutely sure there is no air in system. jacking truck n running heat while burping. that helped a lil but truck still starts to run worse when its warm, lacking power up hills, large temp swings from 195-212. I took out the thermostat. now water flows thru system but it runs too cool 140degrees.
 

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with thermostat out I can drive it a lil farther, but the misfiring is still there. its completely random. it will say cyl 3, I'll clear it. then cyl1,2,3,4 misfire codes pop up. then 2 and 5.... completely random. that's why I dont think its injectors or anything cylinder specific.
I have read many posts about swapped head gaskets blocking coolant jackets. I just dont get why there's no flow in system unless I take out thermostat, but I passed the boil test at 180 degrees exactly? and why radiator is hot on top cold on bottom unless thermostat is out? would that rule out the head gaskets being swapped since I have flow now after removing thermostat?
I have read hundreds of posts about this. I feel like I've checked alot of the main culprits.
I'm about to do a compression test since the new engine overheated on my first real drive of 3 miles. fuel, spark, air, compression. am I missing something? I'll check the cam and crank sensors too
 

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I was hoping it was fuel pressure. it's an old truck so I just went ahead and changed the fuel pump and filter too. runs a lil smoother but still misses and overheats. oh and yes I did verify my plug wires are routed correctly. I've checked 10 times cause this is a real head scratcher.
 
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