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CEL when coasting down hill, or low throttle


Kevin Evans

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I was driving my friend's 93 Ranger 4.0 and I noticed that he gets a check engine light on usually when he coasts downhill (with zero throttle), or sometimes when it's in too high of a gear with little throttle. As soon as the coasting ends, the check engine light goes away.

I tried scanning it with an EEC scanner during KOEO and it doesn't seem to pull any codes (but I'm not sure if I'm doing it right).

I'm guessing the O2 sensors (original sensors) are reading a possibly-incorrect too rich condition, or maybe the AF is just running too rich, dunno. Will have to check the plugs' conditions next time he's over at my house I guess.

Does anyone know that this check engine light is about?
 


chewy012

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Kevin Evans said:
I was driving my friend's 93 Ranger 4.0 and I noticed that he gets a check engine light on usually when he coasts downhill (with zero throttle), or sometimes when it's in too high of a gear with little throttle. As soon as the coasting ends, the check engine light goes away.

I tried scanning it with an EEC scanner during KOEO and it doesn't seem to pull any codes (but I'm not sure if I'm doing it right).

I'm guessing the O2 sensors (original sensors) are reading a possibly-incorrect too rich condition, or maybe the AF is just running too rich, dunno. Will have to check the plugs' conditions next time he's over at my house I guess.

Does anyone know that this check engine light is about?
Click to expand...
Try the fuel pump relay. ~$20... Mine ('92 4.0) use to stick on causing me to run rich. Also, would get stuck off and the truck would die.

When coasting in gear with no throttle, I believe the full system is suppose to shut off to co serve fuel until it get to a threshold rpm where it kicks back to idle values.

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Spott

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chewy012 said:
Try the fuel pump relay. ~$20... Mine ('92 4.0) use to stick on causing me to run rich. Also, would get stuck off and the truck would die.

When coasting in gear with no throttle, I believe the full system is suppose to shut off to co serve fuel until it get to a threshold rpm where it kicks back to idle values.

Sent from my XT1254 using Tapatalk
Click to expand...
That doesn't sound right. The fuel pump and relay should always be energized when the engine is running over ~400 RPM.

You're correct that when coasting with the throttle closed, the PCM shuts down the fuel injectors to conserve fuel, but it doesn't turn off the fuel pump or relay. A stuck fuel pump relay will never cause your engine to run rich, as excess fuel is continually sent back to the tank via the fuel return line already.

The best way to investigate the problem is to scan when the light is on. Make arrangements to have your scanner plugged in, with you operating it in the passenger's seat (maybe get an OBD-II extension cable if needed), recreate the conditions, and see what the PCM has to tell you.
 
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fastpakr

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Agreed. The fuel pump relay doesn't make sense here. It doesn't function as chewy described.
 
chewy012

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Spott said:
That doesn't sound right. The fuel pump and relay should always be energized when the engine is running over ~400 RPM.

You're correct that when coasting with the throttle closed, the PCM shuts down the fuel injectors to conserve fuel, but it doesn't turn off the fuel pump or relay. A stuck fuel pump relay will never cause your engine to run rich, as excess fuel is continually sent back to the tank via the fuel return line already.

The best way to investigate the problem is to scan when the light is on. Make arrangements to have your scanner plugged in, with you operating it in the passenger's seat (maybe get an OBD-II extension cable if needed), recreate the conditions, and see what the PCM has to tell you.
Click to expand...
Hmmm, welp, I had a running rich code, and an intermittent stalling problem when I would let off the throttle. New fuel pump relay solved both problems.

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fastpakr

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Is it possible you mean the fuel pressure regulator rather than the fuel pump relay?
 
chewy012

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No, fpr was good....

I'll say my explanation of the functionality may not be right, but after what I experienced, it was the conclusion I drew.....

Either way, I had a CEL for running rich (intermittent) and a stalling-no fuel problem (intermittent) and both were immediately cured by replacing the fuel pump relay.

I don't know, but, it's an easy $20 check vs. the few hundreds I paid chasing a similar problem.

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stfree

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Kevin Evans said:
I was driving my friend's 93 Ranger 4.0 and I noticed that he gets a check engine light on usually when he coasts downhill (with zero throttle), or sometimes when it's in too high of a gear with little throttle. As soon as the coasting ends, the check engine light goes away.

I tried scanning it with an EEC scanner during KOEO and it doesn't seem to pull any codes (but I'm not sure if I'm doing it right).

I'm guessing the O2 sensors (original sensors) are reading a possibly-incorrect too rich condition, or maybe the AF is just running too rich, dunno. Will have to check the plugs' conditions next time he's over at my house I guess.

Does anyone know that this check engine light is about?
Click to expand...
I'm having this same coasting problem (1990 4.0L Ranger/4x4/auto trans). Codes KOEO 41 (lean). So far have new O2 sensor, new 195f thermostat. New relays. Verified no exhaust leaks, no vac leaks. The CEL only comes on at the end of a 2500' descent from my home (I live at 5600') which takes about 15 minutes. If I continue to drive after that the light will usually extinguish by itself but it takes a while. If I stop, even for just a minute, the light will not re-illuminate on restart. I can drive it all day without issue unless I coast for a long time. It will also idle at either altitude all day without issue. Otherwise runs and idles fine. Any suggestions welcome.
 
gw33gp

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Sounds like it could be an small exhaust leak that is drawing air in while coasting. That's my long shot.
 
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