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95.5-2004 Tacomas & 96-2002 4Runners 4th gen pickups and 3rd gen 4Runners

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Old 09-16-2011, 03:52 PM   #1
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1999 4runner with P0171 and P0303

This is a thread for my dad as it's his car.

He has a Toyota 4runner with 5vz-fe engine, 2wd. It has MIL with codes P0171 and P0303. I've measured resistance with all of the senors mentioned in the FSM. 2 sensors I cannot read correctly are the MAF and Camshaft position sensor. I think I might have my DMM set incorrectly. On my DMM, there's 200, 2K, 20K, 200K, and 2M. Which should it set at?

I've also cleaned out his TB and IACV which did help warm starting issues as other threads have suggested. Currently, when running, his car would drop to low idle around 600-650 and sputters and shakes until around 1.5-2k RPM's. Sounds strange too, maybe an exhaust leak or the typical misfiring sound?

The only things we aren't able to check with a DMM is fuel pressure and compression/leak down. We don't have any testers readily available at the moment. I've read numerous threads which suggested the coil or coil boot but number 3 cylinder's coil and boot are brand new although not OEM. The ohm tested successfully on this coil as well. I'm not sure where else to look. Help?
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Old 09-16-2011, 04:12 PM   #2
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PO171 =" Air/fuel mixture system lean bank 1 (Cylinder number 1 denotes bank 1)
Probable cause: Low Fuel Pressure
Faulty MAF/VAF sensor
Cylinder misfire condition
Faulty ho2/AF sensor
large vacuum leak.
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Old 09-16-2011, 04:18 PM   #3
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Would a vacuum leak cause the engine to rev pretty high or above normal? I don't think we are getting those symptoms. Also, he pulled the plugs today and said they were "wet." At first, he thought it was the IACV valve I cleaned up but we took that apart and it showed no signs of leak. The smell is more on the gassy side of things. The MAF sensor is new OEM from Toyota which has only been on there for a few months. What is a VAF and ho2/AF sensor?
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Old 09-16-2011, 04:20 PM   #4
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Try pulling your maf sensor and cleaning it using maf sensor cleaner, when you clean it there is what looks like 2 thick wires connected at the end with a bulb, that is the air temp sensor you should clean this but more importantly when you look down inside the tube you should see 2 very small wires that is what you want to clean
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Old 09-16-2011, 04:23 PM   #5
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yeah a vacuum leak will cause it run like crap, i ran the PO171 code not long ago, and ended up being a combination of dirty maf, vacuum leak due to cracked vac lines and a bad O2 sensor
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Old 09-18-2011, 08:31 PM   #6
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So here's that I did.

Cleaned the MAF sensor with CRC MAF Cleaner spray. It seemed to have gotten rid of P0171.

I moved coil pack on 3rd cylinder to cylinder 1, and moved fuel injector on 3rd cylinder to number 5. I did this to try and figure out if it was the coil, or the injector causing cylinder 3 to misfire. I also added a treatment of seafoam by way of gas tank, brake booster, and crank case.

The results? Well...not so good. Now I get codes p0300, p0302, p0303 (still). I don't think it's the seafoam's fault. Now about spark plugs; are they pre gapped from NGK? I've read some conflicting information about this. Are they gapped at 1.1mm on each side or is that total?
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Old 09-19-2011, 06:32 AM   #7
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I would also like to add that I replaced the fuel filter just recently, and my dad replaced the TPS sensor a few months back.
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Old 09-19-2011, 02:14 PM   #8
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never trust spark plug pre-gap, check the gap yourself and make sure its right
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Old 09-20-2011, 08:50 AM   #9
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Looks like #3 cylinder is shot. I did a compression test which revealed all cylinders to be at roughly 180psi, the 3rd was at about 145psi.
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Old 10-03-2011, 06:28 AM   #10
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So I finally have the head removed. The valves in cylinder 3 look good, not too much carbon build up. It looks as if it's seating properly as well. The cylinder wall looks to be in excellent shape; no scuffs and cross hatches are still there. But looking at the exhaust side, the valves look to be "wet." I'm thinking a bad valve seal or valve guide. Could those problems cause a drop in cylinder pressure? Did I just find my misfire?
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Old 10-03-2011, 06:28 AM
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1999, 4runner, bad, code, error, maf, p0171, p0303, p0303p0171, po171, runner, sensor, symptoms, toyota, trouble

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