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Check Engine Light and Code 71

Old 04-27-2019, 11:22 AM
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Check Engine Light and Code 71

Hello everyone,

A few weeks ago my check engine light came on ('92 Xtra Cab DLX 4x4 3VZE) for trouble code 71 (EGR system malfunction).

The first thing I did was visually inspect the hoses to check for obvious cracks/holes. I discovered that one of my hoses between the intake and the EGR modulator was indeed cracked, so I replaced it with some new hose and assumed that would end the issue. I pulled the fuse for the computer, the light went off, after a trip or two the light came back one. I disconnected the battery for a more substantial reset of the computer, the light went off, and after a trip or two, the light came back on.

I proceeded to dig into the FSM for further test procedures and read up on other people's experiences here in the forum. Here is what I've done so far:

Note: my truck doesn't have the EGR temp sensor as part of the system
Tested the EGR valve itself for functionality and/or blockage: PASS
Tested all of the VSV's in the system for functionality, resistance, short circuit: PASS
Tested all hoses for blockage: PASS
Tested for appropriate vacuum at the various locations directed by the FSM: PASS
Tested the EGR modulator for blockage and functionality as directed by the FSM (also removed and cleaned it): PASS
Tested the EVAP system's charcoal canister for blockage and blew compressed air through it to clean it out

I've reset the computer half a dozen times or more at this point and the light keeps coming back on, usually after the 2nd trip in the vehicle (approximately 5-10 miles).

My next thought is to pull the EGR valve off of the engine to give it a clean up and remove any major build up on the port.

I'm wondering if there are any glaring omissions from what I've already done that I should be addressing. And, I'm wondering if the trouble code 71 (despite being specific to the EGR system in its description) would be extended to any of the other parts of the emission system such as the EVAP, PAIR, PCV, or Three Way Cat system.

Thanks in advance for any input or advice.
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Old 04-27-2019, 12:05 PM
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If you get code 71 you have an EGR temp sensor, or in the case someone swapped your computer the computer expects this sensor input.

You maybe thinking you don't because you are not in California, this nomenclature is misleading. They produced and sold the California emissions standards outside of California. (Simplicity, costs, ect..)

Do you have an IR temp gun or are you willing to hand test the temperature of the EGR tube where it enters the intake plenum. If it is hot the valve and modulator is presumably working.

EGR temp code 71, is two trip detection. There are very specific instructions in the FSM for the test drive. From memory, you need to be in high gear driving at 55mph for two-five miles, shut off the engine for one to two minutes and repeat the drive.. At your mid test drive pull over spot, hop out and check that EGR pipe it should be hot, very hot not just warm..

Have you done the idle test? (apply vacuum to the EGR valve with the engine idling, it should stall the engine).. If this test fails you either have blockage or the EGR valve isn't moving. Test which one by applying vacuum to the EGR valve and removing the vacuum rapidly you should hear a solid "think" sound as the valve slams shut. If you don't have the think the valve isn't opening and closing properly.

It's possible you have a failed sensor or a dirty plenum.
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Old 04-27-2019, 12:06 PM
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Originally Posted by jordanbailey View Post
.... Note: my truck doesn't have the EGR temp sensor as part of the system...
Hmm. That's a head-scratcher. Code 71 is triggered by the temp sensor. "240 seconds from start of EGR operation. The EGR gas temp. is less than 55C ..." http://web.archive.org/web/201211190...85diagnosi.pdf If you're certain you don't have an EGR Temp sensor, the only thing I can think of is that you have a (replacement) ECM that expects a temp sensor, and is not finding it.

Sensor aside, the output pipe from the EGR valve to the plenum should feel "warm, but not hot." That's pretty imprecise, so that's what the temp sensor measures.

Did you apply vacuum directly to the EGR valve, and detect engine roughness indicating flow? (Step 6 http://web.archive.org/web/201204061...77exhaustg.pdf) At the vintage of our trucks, that valve (and even the part inside the plenum) can get enough gunk to stop the flow, even though everything on the vacuum side is commanding the valve to open.
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Old 04-27-2019, 12:19 PM
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Even without a sensor isn't the wiring still in place for the sensor and it has a plug that goes in the end of it, possibly has a built in resistor. Remember seeing someones ride on here a long time ago in stock configuration that did not have a sensor, but the wire was still there and it had an unusual plug in the end.

Pics

Last edited by Andy A; 04-27-2019 at 12:21 PM.
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Old 04-27-2019, 12:35 PM
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Yeah, it definitely has me scratching my head.

I'm confident the truck didn't come equipt with the temp sensor based on the state of the port where it would've attached to the valve (doesn't look like anything was ever attached there). Here are some pics, https://imgur.com/a/iO4VSoD). It was originally purchased in Oregon and remained there throughout its life. I definitely didn't remove it or change the ECU. I did perform the vacuum test to the valve and the engine does stumble.

To give a bit more context, in the days leading up to this CEL coming on for the first time, I was installing daytime running lights on the vehicle. In order to install the system, I had to find a power source in the engine compartment that was active when the key was in the "on" position. In order to test for that, I was using a multimeter and a test light on several of the connections (circled in the photos). If one of those circled connectors is for the EGR temp sensor, do you think it's possible that by giving power to it via the test light that I could have somehow triggered the ECU to start expecting the temp sensor to be there, even though the truck never had one? I realize this might be a reach, but based on the timing of the light coming on, I am inclined to think that some of my activity in setting up this DRL system caused this issue.
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Old 04-27-2019, 02:17 PM
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Originally Posted by Andy A View Post
Even without a sensor isn't the wiring still in place for the sensor and it has a plug that goes in the end of it, possibly has a built in resistor. Remember seeing someones ride on here a long time ago in stock configuration that did not have a sensor, but the wire was still there and it had an unusual plug in the end.

Pics
Here are some pics. https://imgur.com/a/iO4VSoD
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Old 04-27-2019, 02:33 PM
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Hmm, Wouldn't testing the EGR system without temp sensor give a inaccurate diagnosis?
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Old 04-27-2019, 02:42 PM
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Originally Posted by Co_94_PU View Post
If you get code 71 you have an EGR temp sensor, or in the case someone swapped your computer the computer expects this sensor input.

You maybe thinking you don't because you are not in California, this nomenclature is misleading. They produced and sold the California emissions standards outside of California. (Simplicity, costs, ect..)

Do you have an IR temp gun or are you willing to hand test the temperature of the EGR tube where it enters the intake plenum. If it is hot the valve and modulator is presumably working.

EGR temp code 71, is two trip detection. There are very specific instructions in the FSM for the test drive. From memory, you need to be in high gear driving at 55mph for two-five miles, shut off the engine for one to two minutes and repeat the drive.. At your mid test drive pull over spot, hop out and check that EGR pipe it should be hot, very hot not just warm..

Have you done the idle test? (apply vacuum to the EGR valve with the engine idling, it should stall the engine).. If this test fails you either have blockage or the EGR valve isn't moving. Test which one by applying vacuum to the EGR valve and removing the vacuum rapidly you should hear a solid "think" sound as the valve slams shut. If you don't have the think the valve isn't opening and closing properly.

It's possible you have a failed sensor or a dirty plenum.
I understand what you're saying regarding the temp sensor and the light. This truck is bone stock and I purchased it from the original owner last May. I've driven it without any modifications for the past year and had no issues. Do you have any knowledge of the setups where there isn't a temp sensor present? Someone else in the thread suggested that on these models without the sensor there was a special plug on the wiring harness where the temp sensor would plug in. On my truck, that plug isn't there now, but it's possible that I disconnected it/lost it while poking around for a power source for my daytime running light module.
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Old 04-27-2019, 02:53 PM
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Originally Posted by Andy A View Post
Even without a sensor isn't the wiring still in place for the sensor and it has a plug that goes in the end of it, possibly has a built in resistor. ...
I don't know if this is true. My EWD shows that the 2-pin plug has a BR-B and a G-W wire. Check the wire color on the plugs in your photo. See if you can remember pulling a cap off that plug.

I suppose Andy A is onto something; on the non-California trucks they may use the same ECM and harness wiring, just omitting the sensor and replacing it with a 10k ohm (or thereabouts) resistor. It's possible, I suppose, that your EGR is working and you've just mess with the wiring. In which case you could fabricate a 10k ohm resistor to fit into that connector.

I'm against bypassing any of the sensors, but your EGR valve is not tapped for the sensor. So it looks like you never had one.
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Old 04-27-2019, 03:35 PM
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Originally Posted by scope103 View Post
I don't know if this is true. My EWD shows that the 2-pin plug has a BR-B and a G-W wire. Check the wire color on the plugs in your photo. See if you can remember pulling a cap off that plug.

I suppose Andy A is onto something; on the non-California trucks they may use the same ECM and harness wiring, just omitting the sensor and replacing it with a 10k ohm (or thereabouts) resistor. It's possible, I suppose, that your EGR is working and you've just mess with the wiring. In which case you could fabricate a 10k ohm resistor to fit into that connector.

I'm against bypassing any of the sensors, but your EGR valve is not tapped for the sensor. So it looks like you never had one.
Scope, the harness with BR-B and G-W wire is not connected to anything (it's the large circle in the photo I shared)..... and it's clean, which leads me to believe that I must have disconnected whatever used to be there (and probably lost it). Have you come across a part fiche of EGR system that has variations based on the state. I've seen fiches in the past that indicate CA-only or something like that.
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Old 04-27-2019, 04:12 PM
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Originally Posted by jordanbailey View Post
... Have you come across a part fiche of EGR system that has variations based on the state. ...
No.

Keep in mind that we (at least me) are all speculating about how the ECM works. Before I bought anything like an official plug, I'd try a resistor in the connector. 10k seems to be popular https://www.frys.com/product/7307784, and that's consistent with the temp. curve of the sensor. http://web.archive.org/web/201003261...00egrgaste.pdf

It may or may not be a long term fix for you, but if hooking up the resistor causes the code to go away, you'll know which direction to go.
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Old 04-27-2019, 04:28 PM
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Originally Posted by scope103 View Post
No.

Keep in mind that we (at least me) are all speculating about how the ECM works. Before I bought anything like an official plug, I'd try a resistor in the connector. 10k seems to be popular https://www.frys.com/product/7307784, and that's consistent with the temp. curve of the sensor. http://web.archive.org/web/201003261...00egrgaste.pdf

It may or may not be a long term fix for you, but if hooking up the resistor causes the code to go away, you'll know which direction to go.
i would try what Scope has suggested, 10k resistors are cheap and you should just be able to bend the ends and plug it into the socket. If it cancels out the CEL your headed in the right direction.

I do remember seeing an actual plug that goes in that connector for those trucks that did not come with a temp sensor.

I did find this statement in an old egr delete thread back in 2010

" Also I didn't have a EGR temp sensor in its place is what looks like to me to be a dud sensor with a resistor in it." Posted by Xeo on 8/9/2010

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Old 04-27-2019, 04:43 PM
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Originally Posted by Andy A View Post
i would try what Scope has suggested, 10k resistors are cheap and you should just be able to bend the ends and plug it into the socket. If it cancels out the CEL your headed in the right direction.

I do remember seeing an actual plug that goes in that connector for those trucks that did not come with a temp sensor.
Right, going to find a resistor and get it in there.
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Old 04-27-2019, 05:31 PM
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Originally Posted by Andy A View Post
i would try what Scope has suggested, 10k resistors are cheap and you should just be able to bend the ends and plug it into the socket. If it cancels out the CEL your headed in the right direction.

I do remember seeing an actual plug that goes in that connector for those trucks that did not come with a temp sensor.

I did find this statement in an old egr delete thread back in 2010

" Also I didn't have a EGR temp sensor in its place is what looks like to me to be a dud sensor with a resistor in it." Posted by Xeo on 8/9/2010
Ok. So, the good news is that I was able to put a resistor in there and it seems to have resolved the CEL which didn't come on after two trips of 5+ miles @ more than 55MPH. I'm pretty sure that I misplaced or lost the plug that used to be in that spot. Any ideas on tracking down a part number for something like this? All the part fiches I'm seeing don't make a distinction between vehicles with and without the temp sensor.
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Old 04-27-2019, 06:22 PM
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Might try a site like toyotapartsdeal where you can actually plug in your VIN.
Local dealer also.
Really curious as to the part number for that plug and what it is called.
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Old 04-27-2019, 06:29 PM
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Originally Posted by Andy A View Post
Might try a site like toyotapartsdeal where you can actually plug in your VIN.
Local dealer also.
Really curious as to the part number for that plug and what it is called.
Thanks, Andy. This is what I've found so far and this part looks very familiar to me. https://forum.ih8mud.com/threads/egr.../post-10426198

Part number is: 82824-35020
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Old 04-28-2019, 03:59 AM
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Awesome jordan, hope that does the trick. Thanks for posting up.

Well that sucks, part appears to be discontinued from what I am finding, so unless you find your old one you might be stuck using the 10k resistor.

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Old 04-28-2019, 06:54 AM
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Wow! I actually looked "at" that part yesterday, but the description was "CONNECTOR, WIRING HARNESS" https://parts.lakelandtoyota.com/sho...roduct=4193719 so I couldn't tell if it was the one. And as Andy A says, it's long discontinued. https://parts.lakelandtoyota.com/p/_...282435020.html jordanbailey might well be the first person in 25 years to even think of replacing that part ...
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Old 04-28-2019, 10:35 AM
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Originally Posted by scope103 View Post
Wow! I actually looked "at" that part yesterday, but the description was "CONNECTOR, WIRING HARNESS" https://parts.lakelandtoyota.com/sho...roduct=4193719 so I couldn't tell if it was the one. And as Andy A says, it's long discontinued. https://parts.lakelandtoyota.com/p/_...282435020.html jordanbailey might well be the first person in 25 years to even think of replacing that part ...
Thanks again for the help and insight. I ordered one last night, fingers crossed.

From a knowledge/understanding perspective, what is the difference, functionally, in the EGR system with and without the temperature sensor?
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Old 04-28-2019, 10:37 AM
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This has been an interesting case study.

I wonder if there is a way to tell which trucks had the ECU and wiring for the EGR temperature sensor, but never had the sensor itself installed.

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