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Shuts off, misfiring, code 51?

Tiny
SECOND1UP
  • MEMBER
  • 1990 TOYOTA COROLLA
  • 1.6L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 145,000 MILES
Car was running perfectly then one day (after a super cold night) it started missing bad and idling so badly it will shut off at a stop sign. 30% loss of power. Still runs and drives but not well. Coughs and spits when it started but will even out some after idling a bit. Doesn't shut off idling in the driveway but it does when I drive it. I thought it in the beginning it was using a bit of antifreeze so maybe it was a bad head gasket. Sometimes it blows water out the back but sometimes not. Maybe condensation, but sometimes it does it when it's warmer and not when it's colder outside. Mystery.

I checked compression, 128 even in all cylinders and no bubbles in the radiator. Radiator pressure is normal.

I checked it for codes, and I got 51. This is a switch signal issue, but explanation doesn't help. "A/C signal on, IDL contact off, or shift position in R, D, 2 or 1 range; with check terminals T and E1 connected." I did the proper connection. No problems with A/C, it works. Shifting works, it starts only in N and P. That leaves idle contact. I assume that is TPS?

I checked it out, but I cannot find any instructions online that show the correct TPS. All instructions are for a 4 pin and mine is 3 pin. I can buy the correct one, by picking the one that looks like mine, yet there is another one that is commonly listed but it isn't the right one.

I get voltage to each outside terminal of 11.5 volts intermittently, as I turn the sensor. The center appears to be the source voltage and it switches from one terminal the other. Doesn't make sense to me because I opened it up and it is simply a switch from one side to the other.
Wide open throttle connects one terminal and closed connects the other, that's it. There is no Rheostat. There is no variable voltage in this TPS. I wish I could post a photo. It appears to work as designed, but not according to anything online. The other one they always show should have 5 volts source and vary down to.5 or so. Does anyone know anything about this?
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Sunday, January 22nd, 2023 AT 4:00 PM

12 Replies

Tiny
SECOND1UP
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I was mistaken about which pins have voltage. The two outside pins have 11.5 v when they are not touched by the center contact, but when they are, they go down to 0v. Never seen anything like it in my life.
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Sunday, January 22nd, 2023 AT 4:52 PM
Tiny
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Center pin shows zero always. It's like it is grounding out the other ones when they touch, but no sparks or heat involved.
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Sunday, January 22nd, 2023 AT 4:54 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

Idle speed is controlled by the throttle actuator, but the ECU uses different sensors to determine what is needed. The first thing that came to mind when I read your description was dealing with the idle speed VSV (vacuum switching valve) see pic 1 below. Check those vacuum lines to make sure nothing is disconnected, broken, or dry-rotted.

Next, the throttle position sensor, according to my schematics, does have 4 wires. See pic 2 below and let me know if the wire colors indicated match what you see.

One other thing that can cause issues is if the coolant temp sensor is failing and not signaling the correct temp to the ECU. Based on the signal, it could cause the engine to run extremely rich or lean. So, when it is running, pay attention if there is any dark gray or black smoke from the exhaust or if there are any strange odors.

There were two engines, 4AGE and 4AFE, that were put in these vehicles. By chance do you know which one you have? They are basically the same engine, but one was designed a little differently for performance.

Let me know. Also, check all vacuum hoses for leaks.

Here is a link you may find helpful:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Take care and let me know if you have questions and what you find.

Joe

See pics below.
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Sunday, January 22nd, 2023 AT 7:39 PM
Tiny
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Thank you for the reply. The only smoke is sometimes when I first start it up it does smoke for 10 seconds or so, then quits. Regardless of the schematic, my TPS is 3 prong, so that schematic does not apply to this car. That's what threw me because I can't find any schematic or mention of it online, although I can buy the part. I will post a photo below. The engine is 4A-FE. It's night here but I did take a quick look at the vacuum hoses and didn't see any issues but will look more thoroughly tomorrow.
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Sunday, January 22nd, 2023 AT 10:15 PM
Tiny
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Info and TPS photos. TPS without cover on it. This is not a variable resistor component. Just contacts, that is it.
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Sunday, January 22nd, 2023 AT 10:21 PM
Tiny
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I drove it with the TPS removed and it ran better. Not perfect, but better. More power, smoother, but still has miss.
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Sunday, January 22nd, 2023 AT 10:23 PM
Tiny
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I haven't found any vacuum leaks.
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Monday, January 23rd, 2023 AT 3:25 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

See pic below. The FE engine has a three-pin connector. For some reason, the four-pin is the only one shown in the schematics. If you look at pics 2 and 3, they provide the directions for adjusting the TPS. If the vehicle ran better with it disconnected, I would try this first.

Also, after digging, I found the OEM wiring schematic showing the three-pin TPS. The last two pics are what I found. I had to cut the pic in two to make it readable for you, but I did overlap them.

Let me know if this helps.

Take care,

joe

See pics below.
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Monday, January 23rd, 2023 AT 5:57 PM
Tiny
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I didn’t realize I could post a video. Here is a cold start and idle. Rough at first, then smooths out, but still misses. Because the mike picks up sharp sounds more than muffled sounds, it gets harder to hear at the end, but it is still there. Every few seconds, a miss. This will cause it to shut off at a stop sign with it in drive. Not when idling in Park.
I reinstalled the TPS and adjusted it properly. I turned it about a quarter inch counterclockwise from where it was. It does run better than it was running, but still has the miss. Power is about 85-%. I still don’t know what precipitated this whole episode. It ran fine until one day it didn’t. TPS was tight, has been where it was since I have had the car. It runs way better now than when it first happened and ai haven’t done much, except adjust the TPS.

New plugs, recent coil, checked cap and rotor, they look good, put injector cleaner in tank, no vacuum leaks that I could find. Not sure where to go now. Suggestions?
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Tuesday, January 24th, 2023 AT 10:34 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

I certainly heard it. Was code 51 the only one you found? I heard the misfire so I would think there would be additional codes.

Since it starts running better when the engine warms up, I'm questioning if there is a vacuum leak at the intake or throttle body.

Do me a favor, if you have a vacuum gauge, check the engine vacuum at an idle and let me know what you find. (See pic below)

Also, take a look through this link on how to locate and repair engine vacuum leaks:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know.

Joe

See pic below.
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Wednesday, January 25th, 2023 AT 6:54 PM
Tiny
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I isn't giving any codes since I fixed the TPS. One of the contacts on that was dark, so I sanded it a little. Possibly that was a factor. I don't know, but I don't think so because the voltmeter showed it was contacting.

I don't have a vacuum gauge but can get one. I have found no leaks. Strangely, my 89 Dodge Dakota started displaying similar issues and after much fiddling around with it, I changed the EGR valve, and it purrs like a kitten now. I wonder if that might be the problem here, too.
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Thursday, January 26th, 2023 AT 5:11 PM
Tiny
JACOBANDNICKOLAS
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Anything is possible. I do, however, question the connector you cleaned up. Has it been running differently since that was done?

As far as the EGR, I would think there would be a code related to it.

Let me know.

Joe

I attached two pics below related to testing for the original code. See if anything stands out to you.
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Thursday, January 26th, 2023 AT 7:02 PM

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