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86-95 Trucks & 4Runners 2nd/3rd gen pickups, and 1st/2nd gen 4Runners with IFS

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Old 04-07-2012, 11:53 AM   #1
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1990 3vze Code 51 Under Load

I'm chasing down an erratic idle problem ( usually after driving stop-and-go for a while, the truck will idle as low as 350 rpms ).

I have no stored code at all. I unplugged the EFI fuse to reset the ecu, drove around all morning, and still have no codes.

With the diagnostic pins jumpered, if I hit the accelerator, I get a code 51 that goes away as soon as I let off the gas.

What I can't tell is if this is proper behavior or not. Toyota tells me that running the a/c with the diag pins jumpered with throw a 51 as well.

Again, it is not storing any codes, nor is the CEL light on.

I just dialed the advance down from 12 before to 10 before, and will check the TPS again.
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Old 04-07-2012, 12:14 PM   #2
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Yup, that's what it says. http://www.ncttora.com/fsm/1993/engine/85diagnosi.pdf

IS your A/C on when the idle drops (not the switch, but the compressor itself)? If your A/C idle-up isn't hooked up correctly (seems to happen pretty often) that should cause a big idle drop.
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Old 04-07-2012, 12:35 PM   #3
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It behaves as it should with the A/C on. Compressor clutch locks, rpms go up. Compressor clutch disengages, rpms go down.

The low idle behavior exists, generally, with the A/C on or off after the engine has been at operating temperature for 20 or so minutes and most often in stop and go traffic, or after several rapid, successive starts and shutdowns. Like going to the bank, grocery store and hardware store.

The "running 51" goes when the A/C is on AND when throttle is opened. It goes away when both conditions are not present. EG, no throttle applied, no ac.

I just checked the TPS, and everything seems to be in range. I installed and calibrated it myself in September. No hesitation or bucking. 16mpg in a 3.0 4Runner, occasionally getting up to 17 on summer gas w/o ac.

I was getting 12-13 before I replaced the TPS and distributor.

Advance is now set back to 10btdc.

Interestingly, it just passed emissions with flying colors, except the idle CO, where it barely scraped by with 0.02 to spare. Low idle, rich state and too much advance generally cause that, I think.
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Old 04-07-2012, 01:32 PM   #4
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Quote:
Originally Posted by ToyotaViejo View Post
With the diagnostic pins jumpered, if I hit the accelerator, I get a code 51 that goes away as soon as I let off the gas.

What I can't tell is if this is proper behavior or not.

The "running 51" goes when the A/C is on AND when throttle is opened. It goes away when both conditions are not present. EG, no throttle applied, no ac.
It's working properly. That's exactly what it should do.
Quote:
DIAGNOSTIC TROUBLE CODES OUTPUT
(Normal mode)
To obtain an output of diagnostic trouble codes,
proceed as follows:
1. Initial conditions
(a) Battery voltage above 11 volts or more.
(b) Throttle valve fully closed (throttle position sensor
IDL points closed).
(c) Transmission in neutral position.
(d) Accessories switched OFF.
(e) Engine at normal operating temperature.

2. Turn the ignition switch ON. Do not start the engine.

MALFUNCTION INDICATOR LAMP CHECK

1. The malfunction indicator lamp will come on when the
ignition switch is placed at ON and the engine is not
running.
2. When the engine is started, the malfunction indicator
lamp should go off.
If the light remains on, the diagnosis system has
detected a malfunction or abnormality in the system.
3. Using SST, connect terminals TE1 and E1 of the DLC1.

_________


Code
No. 51

Switch
Condition
Signal

Displayed when A/C is ON, IDL contact OFF(IDL points open) or
shift position in “R”, “D”, ”2”, or ”L” positions
with the DLC1 terminals E1 and TE1 connected.
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Last edited by MudHippy; 04-07-2012 at 01:43 PM.
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Old 04-07-2012, 04:39 PM   #5
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Thanks, MudHippy. That's how I understood it; but I wanted to make sure someone else came to the same conclusion. More eyes on the problem, and all that.

I set the ignition timing back to 10btdc, and replaced the o-ring on the idle adjust screw. The old one was hard and there was basically no resistance in turning the adjust screw. It was previously set to 12btdc. Could this advance have been the reason for the erratic low idle? My understanding is that the ECU adjusts advance as one of only means it has for controlling how the engine is running.

So I warmed up the truck to full operating temperature, jumpered it and set the idle.

I drove it for 15 miles at high speed, low speed and even simulated heavy traffic in my neighborhood. Every time, it returned to idle at 800.

I wasn't able to test "off and on" conditions, but will tomorrow when I go run errands.

I also did not test idle conditions after running the a/c and then shutting it off without turning off the truck.
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Old 04-07-2012, 04:39 PM
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1990, 30, 3vze, 41, 4runner, 51, 89, ac, cel, code, codes, flashing, sensor, shift, stops, toyota, unplugged

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