code 172 + ignition problems
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Junior Member
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code 172 + ignition problems
these problems may be unrelated but any input is appreciated
94 f150 5.0
cel code 172 - HEGO sensor circuit indicates system lean
truck starts perfect when cold, after being warmed up you usually have to crank it a little longer than id like to get it running but always does start
new parts
battery
alternator
plugs
wires
clean MAF sensor
(previous owner says he replaced cap and rotor)
what else can i check/clean?
10mpg on city/highway
94 f150 5.0
cel code 172 - HEGO sensor circuit indicates system lean
truck starts perfect when cold, after being warmed up you usually have to crank it a little longer than id like to get it running but always does start
new parts
battery
alternator
plugs
wires
clean MAF sensor
(previous owner says he replaced cap and rotor)
what else can i check/clean?
10mpg on city/highway
Senior Member
Joined: Nov 2006
Posts: 5,495
Likes: 7
From: Memphis, TN 38135, USA, Earth
What transmission? If you don't know the model, read this caption & follow the links in it:
Why did you replace those parts? Did you test any of them before removing them? Did you test the replacements before OR after installing them? What brands are they & where did they come from?
Obviously, the first thing to check would be: is the truck running lean? Does it run hot? Does it knock/ping/detonate/diesel? Does it get exceptionally good gas mileage? (That's a big NO!
) If none of those, then it's probably not running lean. So then you'd need to find out why the sensor is getting a lean reading...
The most likely cause is an exhaust leak upstream of, or very close downstream from the HEGO. That would also cause the EEC to compensate by enriching the mix, which would result in very LOW mileage (like you have). The fastest way I've found for IDing an exhaust leak is to stuff the tailpipe with rags at idle. Any leak causes an obvious hiss - concentrate on those near the HEGO - those near the muffler or tailpipe are insignificant.
Why did you replace those parts? Did you test any of them before removing them? Did you test the replacements before OR after installing them? What brands are they & where did they come from?
Obviously, the first thing to check would be: is the truck running lean? Does it run hot? Does it knock/ping/detonate/diesel? Does it get exceptionally good gas mileage? (That's a big NO!
) If none of those, then it's probably not running lean. So then you'd need to find out why the sensor is getting a lean reading...The most likely cause is an exhaust leak upstream of, or very close downstream from the HEGO. That would also cause the EEC to compensate by enriching the mix, which would result in very LOW mileage (like you have). The fastest way I've found for IDing an exhaust leak is to stuff the tailpipe with rags at idle. Any leak causes an obvious hiss - concentrate on those near the HEGO - those near the muffler or tailpipe are insignificant.
Thread Starter
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Junior Member
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running lean : dont know
running hot : dont think so
no knocking or pinging that i have noticed
i have an auto trans that came with the 4x4... not sure they made more than one model this year?
the leak would have to be upstream of the o2 yes, i dont think it matters, but everything after my o2 sensor is custom. the rear cat is gone, flowmaster 44 sido to side pipes.
if i stuff rags in my pipes, and there is no leak, is that going to cause worse problems just doing it for 30 seconds or so?
i replaced the parts like an idiot. trying to diagnose an ignition condition where the truck would not turnover right away when warm. plugs looked bad, so i put some bosch plugs in. when changing them i noticed plug wires had gashes here and there and were exposed, replaced with yellow accel wires.
is it very unlikely that it is a faulty o2 sensor? is it possible to test the o2 sensor?
running hot : dont think so
no knocking or pinging that i have noticed
i have an auto trans that came with the 4x4... not sure they made more than one model this year?
the leak would have to be upstream of the o2 yes, i dont think it matters, but everything after my o2 sensor is custom. the rear cat is gone, flowmaster 44 sido to side pipes.
if i stuff rags in my pipes, and there is no leak, is that going to cause worse problems just doing it for 30 seconds or so?
i replaced the parts like an idiot. trying to diagnose an ignition condition where the truck would not turnover right away when warm. plugs looked bad, so i put some bosch plugs in. when changing them i noticed plug wires had gashes here and there and were exposed, replaced with yellow accel wires.
is it very unlikely that it is a faulty o2 sensor? is it possible to test the o2 sensor?
Last edited by siberianmist; Feb 5, 2011 at 05:24 PM.