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Old 11-30-2016, 05:59 PM   #1
zipzap8
 
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Default Mystery Issue with 1986 Hardbody (Sunrader)

1986 VG30i Running Rich under Load
ECU Codes 24 and 31
Does anyone else have experience with this?

The truck in question is a 1986 hardbody with the 3.0 V6. It's a motor home chassis which means it had a dual rear wheel axle configuration.

The truck came in running rough at idle and running very rich. When applying throttle the truck would sputter even worse and blow brown/black smoke out the back (raw fuel).

Owner states that they were on a trip heading back home when the truck bogged down and became very difficult to drive - no power and sputtering.

Owner limped truck home and had a mobile mechanic come look at vehicle and he replaced the timing belt, spark plugs, distributor cap and rotor and injectors.

Mobile mechanic unable to reassemble truck properly.

Owner called second mobile mechanic to assess situation. Second mechanic found several small issue including timing off by a few teeth but was able to get the truck running enough for customer to bring it in to the shop.
  • We noted that the timing belt was again off by two teeth. We suspect this was due to the inferior quality timing belt that the first mechanic used. The valves were not damaged since the timing was only off slightly. We swapped out the belt and water pump with quality parts and reassembled.

    We also replaced the spark plugs with the correct plugs and put on new high quality wires.

    Truck still ran poorly.
    .
  • Found that mass air flow sensor was missing a portion of the coating around the hot wire. Replaced the mass air flow sensor and truck ran much better at idle; however, it would still stumble on acceleration.
    .
  • Checked cylinder head temp sensor and monitored while truck warmed up. Readings seemed normal throughout range.
    .
  • Checked oxygen sensor function, oxygen sensor was not responding so we replaced the sensor and tested again for response. Sensor now responding and again there was a small improvement but the struck was still struggling when throttle was applied and running rich.
    .
  • We checked the throttle position sensor (TPS) and were getting erratic readings. TPS would function fine for the first third of pedal travel and then would peg at maximum reading anywhere during the last 2/3 of pedal travel. Throttle position sensor was replaced. Again, a small improvement was noted and it was now in good enough running order to road test vehicle. Truck would run fine for the first third of pedal travel and then bog down and sputter while blowing brown smoke out the back.
    .
  • We checked the injectors and found that they were the incorrect part number. After speaking with owner, we learned the injectors were out of a 4 cylinder truck as that is all the first mechanic could find at the wrecking yard. Replaced the injectors with proper injectors (one new and one rebuilt) and rebuilt throttle body. All gaskets changed and ports cleaned. Found that both idle solenoids missing internal plungers. Found replacement solenoids and replaced both. Fuel pressure regulator also replaced. At this point, all components on throttle body have been replaced.
    .
  • Ignition timing was checked and checked again.
    .
  • We found a corroded ground connection on the engine.
    .
  • We removed the wires and cleaned the corrosion and reassembled.

    Truck still runs poorly under acceleration but idles perfect.
    .
  • Replaced ignition control module as a precaution - no change in performance.
    .
  • Tested back pressure in exhaust for possible plugged cat, no back pressure present.
    .
  • Pulled codes from computer. Codes 24 and 31 keep coming up, code 24 = Transmission Switch (because we did not run the gear selector through the range of gears when pulling codes) and Code 31 = Engine control module (ECM).- A/C, light, fan and PS switch.

    The code 31 is what we have been primarily focusing on. We performed the pin test at the diagnostic port under the dash to make sure the load items (P/S, HVAC Fan, ETC) were sending signal, which they are.

    We suspect at this point that the ECU is faulty.
    .
  • Another helpful piece of information.... this truck has a feature where you can push the "Power" button on the end of the gear selector for towing or heavy duty loads and the "Power" light will illuminate on the dashboard.

    We did notice that when that feature is off and the truck is under heavy throttle, the light will come on and then turn off.

    This indeed makes us suspicious that the ECU is the culprit.
Any input from anybody with experience with this is greatly appreciated!

Last edited by jp2code; 12-01-2016 at 10:53 AM.
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Old 11-30-2016, 09:12 PM   #2
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Did anyone check the timing belt tensioner?
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Old 11-30-2016, 10:23 PM   #3
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what a read. That'll be a nice bill at the end of the day. Are you sure you want to find the problem, you could live off that truck.
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Old 12-01-2016, 11:03 AM   #4
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First, let me say your shop has done a great job on this old truck! It sounds like you are fixing a lot of neglected items.

The 1986 trucks were pre-Standardized OBD1, so the codes are not quite what you think.

Code 31 = Load Switch.

Mine doesn't have a Load Switch, so I don't know what all that involves.

This is from the [VIDEO] Checking Error Codes:



I don't have a Load Sensor. I, naturally, would have ass-u-med that means the ECU has a problem "under load" ...until you mentioned it had a Load Switch.

From what I have heard from others who have these switches (in newer trucks like 1990 and up), turning this switch ON keeps the engine shifting at a higher RPM. With the switch OFF, the ECU controls this operation on its own. So, if you floor the pedal to accelerate faster, the ECU could switch the truck into "Load Mode" and turn the switch ON. I don't know. Mine has a manual transmission.

I'd like to see some pictures of this old beast, if your shop will let you.
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Old 12-01-2016, 01:48 PM   #5
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Oh, I forgot this: Fusible Links!

Link: http://www.infamousnissan.com/forum/showthread.php?t=35558

Those are around $12 at the Nissan dealership. A burned fusible link does NOT throw a code, and can lead to mysterious problems.

You've spent a lot on new parts already on this truck. If the fusible links look old, I'd say go ahead and replace them.

BTW: I love the Heavy Duty Nissan Hardbody trucks like you've got in your shop!
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Old 03-02-2017, 03:28 AM   #6
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If the fuel pressure regulator is vacuum controlled make sure the vac line is not drinking fuel.

Also if you have a smoke machine I would want to hook it up and check for vacuum leaks.

If your lucky enough to find a ecm that matches and is affordable I would give that a shot too.

Seems like its either being force feed too much fuel or it thinks its lean and going rich.
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Old 03-02-2017, 05:01 AM   #7
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they've either fixed it or they've ghostied it into a police station by now
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Old 03-05-2017, 12:44 AM   #8
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Still working on it, actually :/
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Old 03-05-2017, 10:20 AM   #9
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I second the vac line open somewhere, but as JP said replacing the fusible links is relatively cheap. Did your shop do a compression test on each cylinder?
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Old 03-14-2017, 01:15 AM   #10
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Have a compression test and a leak down test been done?

You did not say you did this, but I would recommend replacing the distributor cap and rotor if you have not already.
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