Erratic low idle, code 173
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4wd Low
Joined: Mar 2025
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Erratic low idle, code 173
Guys, I recently bought a '94 F150 with the 4.9L I6 and the E4OD transmission. I bought it cheap and it needs a little work, so I've been fixing it up as I can. The problem I'm having now gives the following symptoms:
1) Idle lowers and bobbles intermittently. It seems to idle good and smooth with a load like idling in gear. The idle problems happen in Park or Neutral and seem to get worse the
longer it idles. It hasn't gotten bad enough to stall yet.
2) The speedometer becomes erratic above 75 mph and jumps around strangely reading withing 5-10 mph of the actual speed.
3) I ran the codes using a jumper wire with the following results: KOEO-629, 512; KOER-173, 632
I've cleaned the EGR, IAC, and throttle body. I ran a can of Seafoam thru a tank of 17 gal of gas. The truck had been sitting for a while before I bought it and the previous owners only averaged driving it about 3200 miles a year. It has 105,000 miles on it now.
Any ideas on what might be happening?
1) Idle lowers and bobbles intermittently. It seems to idle good and smooth with a load like idling in gear. The idle problems happen in Park or Neutral and seem to get worse the
longer it idles. It hasn't gotten bad enough to stall yet.
2) The speedometer becomes erratic above 75 mph and jumps around strangely reading withing 5-10 mph of the actual speed.
3) I ran the codes using a jumper wire with the following results: KOEO-629, 512; KOER-173, 632
I've cleaned the EGR, IAC, and throttle body. I ran a can of Seafoam thru a tank of 17 gal of gas. The truck had been sitting for a while before I bought it and the previous owners only averaged driving it about 3200 miles a year. It has 105,000 miles on it now.
Any ideas on what might be happening?
Logistics Pro
Joined: Nov 2013
Posts: 3,930
Likes: 1,201
From: Mojave, Ca
Guys, I recently bought a '94 F150 with the 4.9L I6 and the E4OD transmission. I bought it cheap and it needs a little work, so I've been fixing it up as I can. The problem I'm having now gives the following symptoms:
1) Idle lowers and bobbles intermittently. It seems to idle good and smooth with a load like idling in gear. The idle problems happen in Park or Neutral and seem to get worse the
longer it idles. It hasn't gotten bad enough to stall yet.
2) The speedometer becomes erratic above 75 mph and jumps around strangely reading withing 5-10 mph of the actual speed.
3) I ran the codes using a jumper wire with the following results: KOEO-629, 512; KOER-173, 632
I've cleaned the EGR, IAC, and throttle body. I ran a can of Seafoam thru a tank of 17 gal of gas. The truck had been sitting for a while before I bought it and the previous owners only averaged driving it about 3200 miles a year. It has 105,000 miles on it now.
Any ideas on what might be happening?
1) Idle lowers and bobbles intermittently. It seems to idle good and smooth with a load like idling in gear. The idle problems happen in Park or Neutral and seem to get worse the
longer it idles. It hasn't gotten bad enough to stall yet.
2) The speedometer becomes erratic above 75 mph and jumps around strangely reading withing 5-10 mph of the actual speed.
3) I ran the codes using a jumper wire with the following results: KOEO-629, 512; KOER-173, 632
I've cleaned the EGR, IAC, and throttle body. I ran a can of Seafoam thru a tank of 17 gal of gas. The truck had been sitting for a while before I bought it and the previous owners only averaged driving it about 3200 miles a year. It has 105,000 miles on it now.
Any ideas on what might be happening?
2) The ABS computer is known to cause speedometer problem, as they age, components inside the computer fail and mute the speed sensor signal coming from the rear differential. It's very easy to disconnect the computer temporarily to see if that's causing the problem.
3) 629-possibly related to the speedo problem.
512- your battery was disconnected in the last ~10 starts.
173- your o2 sensor is lazy and needs to be replaced, or you have an exhaust leak, vacuum leak, or fuel injector leak. This may be related to your idle issue. Start the engine and pull the vacuum line off your fuel pressure regulator. If fuel comes out of the nipple, then the diaphragm is ruptured and the regulator needs to be replaced.
632- you didn't press the od-cancel button during the KOER test.
Thread Starter
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4wd Low
Joined: Mar 2025
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Proto,
Thanks for the advice. I've checked most of the vacuum lines, but I'll focus on that more. I'll disconnect the ABS computer and see what that does for the speedo too. I haven't checked the PCV yet. I've noticed that the air filter has oil on it above where the crankcase vent line comes in. Is that normal?
Thanks for the advice. I've checked most of the vacuum lines, but I'll focus on that more. I'll disconnect the ABS computer and see what that does for the speedo too. I haven't checked the PCV yet. I've noticed that the air filter has oil on it above where the crankcase vent line comes in. Is that normal?
Thread Starter
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4wd Low
Joined: Mar 2025
Posts: 11
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I replaced the PCV valve and unplugged the ABS module. The speedometer works fine now and the idle is much improved, but it still hunts a little every now and then when warm and in N or P...not when it's in gear for some reason. I'm thinking the IAC is sticking open a little. I took it apart and cleaned it and noticed when it is "Closed" I can still blow air thru it. I'm not sure if that's normal or not.
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