I've been trying to fix a code 51 "lean running engine condition".
I have a constant 49PSI at the fuel rail, replace fuel filter anyway, and ran FI cleaner.
Replaced MAP/IAT and 02 sensor.
Replaced plugs and wires.
Cleaned TPS and Throttle body.
New intake and filter.
Replaced valve cover gasket and spark plug tube grommets.
I can't find a vacuum leak, engine vacuum is at 17hg but flutters just slightly very fast.
Which indicates a possible ignition fault, so I hooked up a timing light to each wire.
I got a steady light pulse on cylinders 1 and 4, on cylinder 2 flash missed a beat occationally, and cylinder 3 flashed much faster than any other. Both 1 and 4 increase steadily if I give it gas, both 2 and 3 cut out and fluctuate if I rev it up a bit.
So I tested primary ohm resistance from middle pin to the 2 outside pins and got .9 on both sides. Haynes says it should be .35 to .65. (but that is at 70 degrees and it is over 90 here.) Secondary resistance was 13.1 on C1-4 and 13.3 on C2-3 Haynes says it should be between 11.5-13.5 so that is OK. Seams as though the coil is testing ok, but not preforming right maybe...?
I have to add, the connector of the Coil pack was broken when I bought the car, so I replaced it with one from a junkyard, but the connectors were spade instead of round like the original. I couldn't find the right one, so I just cut of the old connector and wired in the newer style connector with the newer style junkyard coil. I'm thinking this shouldn't be an issue though.
Car had a code 51 before and after this.
Question: What would make cylinder #2 & #3 to misfire other than the coil pack...?
I'm guessing a bad coil pack misfiring 2 cylinders would explain the code 51, and my horrible gas milage. But is there anything else I can check before replacing the coil again? Does the PCM control any of this, or could it be the feed wire to the coil for C2&3 be grounding out somewhere? I can't find it if so... Where do these wires get their power, so I can check continuity in these coil wires?

 

this is what i got for 51

"51 Oxygen sensor stuck at lean position (Bob Lincoln wrote: may be tripped by a bad MAP sensor system causing a rich condition, and the O2 sensor trying to compensate. The O2 sensor may still be good. The MAP assembly consists of two pieces, the valve and the vacuum transducer (round plastic unit with cylinder on top and both electrical and vacuum connections) - If you get hot rough idle and stalling, especially on deceleration, accompanied by flooded engine and difficulty restarting, that can be a bad MAP sensor causing the O2 sensor to try to compensate. If you get poor cold driveability, stumbling and bucking, and acceptable warm driving with poor gas mileage (a drop of 10 mpg or more), that is usually the O2 sensor. [Webmaster note: MAP sensors seem to die regularly.] "

http://www.allpar.com/fix/80s-codes.html

Primary should be .45 to .65 OHMs accros the center and corriesponig cylinder tang (one is for 1&2 the other is 2&3 both fire at the same time) then the secondary should 11,000 and 14,000 ohms.
MSD sells a coil cheaper than OEM.

http://www.modernperformance.com/pro...7e163d2f93e5da

 

I forgot to subtract the ohm resistance of .3 for my DVM. So the coil passes both primary and secondary tests. I searched the site and found the pinout to check the wiring. Hopefully I can find the problem there.
I did already replace the 02 and MAP/IAT sensor. The MAP/IAT failed voltage test replacing it stopped the MAP code. 02 sensor just looked so old, I replaced it to be sure.
The car starts fine, doesn't die on me, it actually idles fairly well maybe a slight stutter here and there, not consistent. And it drives and shifts smooth if you drive like normal. If you try to give it more than 2/3 throttle it surges up and down bad, but doesn't die, and if I back off the gas it runs fine again. My mpg is really low- 12.9. So I doubt it is actually running lean, it's probably running rich to compensate for whats wrong.
When I get to tearing apart the wiring and testing, I'll post results.

 

did you replace both o2s

 

I tested the downstream o2 sensor and got 6 ohms at the 2 white wires, it looked fairly new, compared to the upstream o2 also, so I didn't replace that one.

Before I tore up the wiring harness I wanted to verify ignition failure, so I pulled one plug wire at a time while running, and every one produced a steady spark, and the same poor idle. So it seemed to be fine. So I skipped the wiring and rechecked the downstream 02 sensor. Still the right ohm reading, but this time I checked the power wire of the plug for voltage while running and there was none. And unplugging the downstream 02 didn't affect the running condition at all.

So I ended up tearing up wiring after all, there were a lot of melted wires for the 02 sensor and some other sensor down there, so I teadiously repaced about 18 inches of wire 6 times, crimping and shrink wrapping, and then taped it up real good.

Reset computer and took it for a test drive. What a difference. It idles so smooth now, and I can give it full throttle with no surging.

Thanks for pointing me in the right direction. I'll have to drive for a while and make sure the code doesn't come back. And fill up, drive and fill up again to figure out the gas milage, but I suspect it will be much improved.

Thanks again for your help, Have a great weekend!

 

from what i understand the downstream is only for detecting Cat failure

 

Final report: my gas mileage is up to 21.7mpg, and still no check engine light.