1992 With Some Codes

[Leftysdad]

Recently acquired a 92 wd21 that's automatic with the 3.0.....been parked a year....don't know any history(owner passed),  it will crank and run, but poorly....codes are 

12 / 21 / 34 / 51

Idles very low, with a skip/miss when accelerating,  only thing I've done is unhooked the 2 prone coolant temp sensor,  and it will idle higher,  but still runs poor....any direction of where to start would be appreciated,  thanks in advance 

[Frenchy]

Best advice I can give you is to get a hold of a factory service manual. It will give you all the information you need in order to diagnose this

[peejay]

Agree with Frenchy, the factory service manual will prove invaluable (you "might" be able to find a copy online, post back here if you can't).

 

In the meantime:

 

12 = Mass air flow sensor circuit

21 = Ignition signal circuit

34 = Knock sensor circuit

51 = Injector circuit

 

For the 34/knock sensor circuit, if the knock sensor is in fact faulty, I would recommend relocating it to the top of the intake manifold so that future service/diagnostics are easier. (It's buried underneath the intake manifold and is a pain to get to). There are lots of discussions/writeups detailing how to relocate it.

Edited April 28 by peejay

[Slartibartfast]

Your best bet is the '90 manual. Free download from cardiagn.com last time I checked, or message me if you can't find it and I'll get you a link. The '94/'95 manual from Nicoclub should be close, but it's for the round-dash trucks, and there are some minor changes on those that can hang you up.

 

The EF&EC section has a walkthrough for troubleshooting each code. I'd start with the ignition signal, injector circuit (that'll be the misfire), and MAF codes, and see if the knock sensor code goes away on its own once those three are resolved (clear the codes and see if it comes back). If the engine has no idea how much air is coming in, one cylinder isn't firing, and the timing is messed up, it may well be knocking, and annoyingly these don't have separate codes for a bad knock sensor vs actual knocking.

 

I would also look for any obvious rodent damage. An injector circuit code could be due to a chewed wire, and my '93 had a MAF code when a rat's nest in the air filter got sucked into the MAF.

[peejay]

18 hours ago, Slartibartfast said:

Your best bet is the '90 manual. Free download from cardiagn.com last time I checked, or message me if you can't find it and I'll get you a link. The '94/'95 manual from Nicoclub should be close, but it's for the round-dash trucks, and there are some minor changes on those that can hang you up.

 

The EF&EC section has a walkthrough for troubleshooting each code. I'd start with the ignition signal, injector circuit (that'll be the misfire), and MAF codes, and see if the knock sensor code goes away on its own once those three are resolved (clear the codes and see if it comes back). If the engine has no idea how much air is coming in, one cylinder isn't firing, and the timing is messed up, it may well be knocking, and annoyingly these don't have separate codes for a bad knock sensor vs actual knocking.

 

I would also look for any obvious rodent damage. An injector circuit code could be due to a chewed wire, and my '93 had a MAF code when a rat's nest in the air filter got sucked into the MAF.

Slart, would you mind demystifying the knock sensor operation?

 

It was my understanding that when the sensor senses knock (for whatever "normal" reasons), the sensor sends a signal to the ECU to retard the timing to eliminate the knock. Does this scenario always produce a code/check engine light, letting the driver know that "uh-oh, something is causing knock, better check it out"? Or is the system "supposed" to only trigger a code/light if there is an issue with the knock sensor/circuit? Thanks!

Edited April 29 by peejay
(typo)

[Slartibartfast]

Looks like I was wrong about the knock sensor, and Peejay is correct--code 34 is for electrical faults only. The computer does not throw a code for actual knocking.

 

The electrical checks for the knock sensor are pretty simple. Might as well chase those down while you're after the other three. I suspect you'll find a common fault behind all four--maybe a shared ground that's come loose/corroded, maybe harness damage.

 

Come to think of it, the knock sensor grounds to 35M. The MAF grounds to 35M. The transistor for the coil grounds to 34M, which is right next to 35M. If those are loose/corroded/chewed, that could be three of your four codes right there.

 

The harness diagram in the '90 manual is just this side of useless (they tried to show three different engine options in one diagram). The knock sensor diagnostics in the '95 manual show 34 and 35M near the coil, but I don't see any grounds in that neighborhood on my '93. I think 34 and 35M might be the two at the front of the upper intake, on either side of the Allen bolt, near the coolant temp sensors. Loosen, clean as needed, retighten, clear codes, see if that gets you anywhere.

 

The injectors are grounded through the computer, so they shouldn't have anything to do with those two ground points. The same mouse might've chewed on that harness too, I guess. Or maybe the previous owner parked it because an injector was dead.

 

12 hours ago, peejay said:

Slart, would you mind demystifying the knock sensor operation?

 

Looks like your understanding of it was bang on! But here's the long version.

 

The knock sensor is piezoelectric, similar to a guitar pickup or a microphone. It allows the computer to "listen" to the engine. If the computer "hears" pinging/knocking, it retards the ignition timing to prevent engine damage. It does not set a code when it does this. I don't know how long it waits to reset the timing back to normal afterwards.

 

EF&EC-72 ('90 manual) says that code 34 is set when the knock sensor circuit is open or shorted. The diagnostic procedures for code 34 ('90 and '95) and P0325 ('97) are just electrical checks on the sensor and its wiring. Looks like the computer also pulls timing when that code is set. I guess it's playing it safe while it can't tell if the engine is pinging or not.

 

The '90, '95, '97, and '03 manuals all make clear that the knock sensor is not used in normal operation. It's just there to save the engine when something else goes wrong. Sorta like a smoke detector.

[peejay]

Excellent info as always, slart, thank you!

[level9]

I'd siphon out the gas and replace it along with a bottle of fuel injector cleaner before doing anything else. Gas goes bad. Anything older than 6 months should be tossed; otherwise you can experience some of the issues you are seeing. Even if it's not the issue I wouldn't start trying to diagnose problems while running on old gas.

[peejay]

2 hours ago, level9 said:

I'd siphon out the gas and replace it along with a bottle of fuel injector cleaner before doing anything else. Gas goes bad. Anything older than 6 months should be tossed; otherwise you can experience some of the issues you are seeing. Even if it's not the issue I wouldn't start trying to diagnose problems while running on old gas.

Ditto on that.

 

"Easy button" is disconnect the fuel supply hose from the rail and place the end in a fuel container, pull the fuel pump relay, and run 12V straight to terminal 5 to let the fuel pump do the work for you. Be sure to go back with a new fuel filter, too.