Engine Light and Error Codes
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Joined: Apr 2001
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Engine Light and Error Codes
I've got a 92 F150 with the 4.9L engine and 5-speed manual tranny. The check-engine light has started coming-on over the past 200 miles, intermittently at first, and now all of the time. It doesn't come-on until the thermostat first opens (i.e. the truck is warmed up). Also, the idle has become uneven, cycling up and down several hundred rpm. I ran out the codes, and had a 172 (HEGO reads system lean), and a 327 (EGR valve position circuit below min voltage). The truck was obviously running very rich (I could practically watch the gas gauge go down, and it smelled very rich), and since the O2 sensor was the original, I replaced it. I ran the truck for 10 miles, disconnected the battery for an hour, and drove it again. Same symptoms (engine light, running very rich, uneven idle). This time I'm only getting the 172 code. Any ideas where to start? The truck seems to run fine (aside from the idle) and have plenty of power. I'm pulling my hair out on this one.
Post Fiend
Joined: Nov 2002
Posts: 9,987
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From: Memphis, TN
Engine Light and Error Codes
You shouldn't unhook the battery - just pull the jumper wire while the codes are flashing:
http://www.broncodata.com/tech/codes.htm
Get a Haynes manual and concentrate on the EGR valve.
http://www.broncodata.com/tech/codes.htm
Get a Haynes manual and concentrate on the EGR valve.
Elder User
Joined: Sep 2002
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From: US
Engine Light and Error Codes
I found a good web site for serching code meanings. http://dalidesign.com/hbook/index4.html . Check the list of 3 digit codes. Try changing the egr position sensor ( ontop of egr valve).
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Joined: Feb 2002
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Engine Light and Error Codes
I hear ya on the pull your hair out thing. Sounds like you got the same problem I do. (Read my post - 92 F-150 won't idle). Just don't waste your $ replacing parts like we did, cause it didn't help one bit! If you find a solution, please share. Thanks.
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Engine Light and Error Codes
Hey all - thanks for the suggestions. This morning I pulled the spark plugs (they had a little over 30 kmi on them): they were awful. Very badly eroded, each one opened at least 20 mils. Replaced them, replaced the air filter and crankcase vent filter, (all things I SHOULD have done right of the bat), and life is good. No more engine light, no more error codes, and a steady idle. I guess I should pay a little more attention to the maintenance schedule...
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Engine Light and Error Codes
Well, I spoke too soon. The engine light with 172 error code is back after a brief absence (from both stored memory and the KOER tests). So far (after the new 02 sensor, plugs, and air filter), I've replaced the distributor cap & rotor, fuel filter, and EGR valve position sensor (it seemed notchy and dirty). I also pulled, cleaned, and tested the EGR valve (reinstalled with a new gasket), tested the TPS (no noticeable dead spots), and the MAP sensor (seems to hold vacuum, and change output frequency with vacuum). While I was under the hood, I also pulled the IAC and cleaned it up good. The truck sure runs smoothly now, but taunts me with the codes. The top end power MIGHT be off slightly, but I can't really tell for sure. My bag of tricks is running out. Next I'm planning on testing the fuel pressure, and replacing the PCV valve. Anyone else have any thoughts? Anyone ever hear of a bad O2 sensor out of the box?
- Nate
- Nate
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Posting Guru
Joined: Oct 2002
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From: Dededo, Guam
Engine Light and Error Codes
It's a possibility. I bought a brake master cylinder, followed directions on how to install it. Anyway, pulled the plastic caps off to connect the brake lines and the thing leaked all the fluid in about 15-20 seconds. Made me mad because i had to wait til the next day for the parts store to open up. So yes, it is possible to have parts bad out of the box.
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Engine Light and Error Codes
Well, I replaced the PCV valve (no change), and checked the fuel pressure (well within spec on both tanks). I also replaced the plug wires and coil, since I had some corrosion on the leads (and apparently some arcing). I also replaced the TFI module, in case the resistance and arcing in the wires had damaged it. No change. I even borrowed a data logger and recorded the output from the O2 sensor. The 02 sensor looks like it's working fine, and seems to be reporting rich more than lean (?!?!). I've checked the inlet air temp, and engine coolant temp sensors. No sign of vacuum leaks anywhere. Could it be a bad computer? Do they fail all at once, or do they just start acting up? My smog check is coming up, and I'm feeling the pressure.
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Engine Light and Error Codes
I just wanted to provide a wrap-up on this thread, in case anyone else has a similar problem. I pursued troubleshooting this problem using the Ford Emissions Manual for several weeks, without getting anywhere. I finally broke down (no pun intended) and took the truck in to an independent tech, who was able to monitor the computer's bus. The bottom line was that the 02 sensor was correctly reporting to the computer, but the computer was completely ignoring it (believing that the 02 sensor was always reporting 2 mV - a full lean condition). The truck went rich because of this (which the 02 sensor correctly reported as a voltage of 600 mV - which the computer ignored). Bottom line: it was a bad computer. The tech was skeptical at first: he'd only seen one bad Ford computer before (they are apparently pretty reliable). So I'm back on the road, passed emissions testing, and haven't had any more problems. It sure ran nice when it was rich!
