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Thread: "Check Engine light" flashes when going down a steep hill

  1. #1
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    Default "Check Engine light" flashes when going down a steep hill

    This is a really strange one. I've asked around at other places and nobody ever heard of it... I searched all around, and can't find anything.

    The compression is a little low on my #2 cylinder, and I got the code P0302, which means misfire on the #2..

    But the Check Engine light acts strange...

    There's a really steep hill around here, and the light flashes every time when I'm going down...

    When the road levels out, it stops flashing...

    When that light flashes, it means "hard misfire"...

    But, since I'm not touching the gas "at all" on the way down this hill, I'm trying to add it all up...

    She goes up the hill just fine, but, coming down that hill gets the light flashing every time.

    Anyone ever seen this before? It's a 1998 F-150, 4.2L...

  2. #2
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    Flashing check engine light usually means a more serious problem than a steady light. The miss fire could be because of a valve lifter problem and that could be exacerbated by low engine oil level that lets the pump run dry if the oil pick up is exposed to air when the front of the engine is lower than the back end. Check oil level.

    There is a very slim chance that all that could come together but it's possible. Are you going down the hill when the engine is cold like first trip in the morning? That points to a stuck lifter that works after the engine oil heats up.

    I'm out of ideas now, I'll post again if anything pops into mind as the day goes on.

    Bob K

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    Is it a manual transmission and 2 wheel drive? If so and I'm really stretching here but check to see if you have a bad u-joint in the drive shaft. The reason I say that is that the way the engine detects a miss fire is by watching the speed that the crank shaft is spinning. Going down hill the wheels are forcing the engine to turn over. If there is slop in a u-joint then engine speed may change minutely on every drive shaft revolution, but the crank position sensor will see those little speed changes and think it's caused by a miss fire. I know that's stretching probability quite a bit but still worth looking at. Try going down the hill in a lower gear and see if that changes the light. Out of ideas now.

    Bob K

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    Quote Originally Posted by Bob K View Post
    Is it a manual transmission and 2 wheel drive? If so and I'm really stretching here but check to see if you have a bad u-joint in the drive shaft. The reason I say that is that the way the engine detects a miss fire is by watching the speed that the crank shaft is spinning. Going down hill the wheels are forcing the engine to turn over. If there is slop in a u-joint then engine speed may change minutely on every drive shaft revolution, but the crank position sensor will see those little speed changes and think it's caused by a miss fire. I know that's stretching probability quite a bit but still worth looking at. Try going down the hill in a lower gear and see if that changes the light. Out of ideas now.

    Bob K
    Thanks Bob

    It's a 2WD, automatic... I think the u-joints are pretty good, no noises or clunks, but I haven't checked it close in a while.

    What you said about the valves earlier though, that one has me curious.

    When I did the compression test, #2 was the low one @139. When I put a little oil in there, it only came up to 150. The highest readings on the others were up around 175-180. So yeah. I think it has something to do with the valves.




    I did check the oil when it first happened. It was about half quart low, so I topped it off. But the same thing happened again next time I came down that hill.

    And yeah, it's not fully warmed up when I come down that hill. It a house where I was working at, so the engine is cool when I'm heading down in the afternoon.

    It seems strange that the light doesn't flash on the way up though... A few years ago, I had the flashing light and it was really bad. The pinging was loud and there was no power. The codes showed 2 or 3 cylinders misfiring back then. I replaced the coil pack, plugs, and wires, and that fixed it right up.

    Thanks again Bob
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    Quote Originally Posted by Bob K View Post
    Is it a manual transmission and 2 wheel drive? If so and I'm really stretching here but check to see if you have a bad u-joint in the drive shaft. The reason I say that is that the way the engine detects a miss fire is by watching the speed that the crank shaft is spinning. Going down hill the wheels are forcing the engine to turn over. If there is slop in a u-joint then engine speed may change minutely on every drive shaft revolution, but the crank position sensor will see those little speed changes and think it's caused by a miss fire. I know that's stretching probability quite a bit but still worth looking at. Try going down the hill in a lower gear and see if that changes the light. Out of ideas now.

    Bob K
    hmmm, thanks again Bob... now that I think about it, the timing chain could affect that too, eh?

    There's a definite rattling sound at idle, and it's been there for along time. My best guess is it's just the timing chain rattling around, but I dunno. It could be in the valve train too..

    I don't know how the sensors work, but that sounds like it could possibly have something to do with how that hill triggers the flashing light.

    So far, I've only gone down that hill 3 times... it's a ridiculously steep hill, and if we had snow out here, nobody ever would've put a street there... But anyways, I tried it with the OD turned off, as well as on, and it still triggers the light every time. I never even tried it in second gear...

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    Here's a comment I found online about the valves... this might be worth a try, to see if it helps the compression.

    ~Well my 97 4.2 has 250 thou on it and rattled like a paint can.
    The rocker arms were totally worn down about a 1/16
    I replaced the lifters and push rods and the rocker arms and it cost me about $225.00. Of course I did it myself but now it purs like a kitten.
    Hope this helps but pull off a rocker and looks see how worn it is where the guide fits in it. You may be surprised;I was~








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    [QUOTE=Jim2;341608]Here's a comment I found online about the valves... this might be worth a try, to see if it helps the compression.


    Because it's has an automatic transmission I don't thing anything from the drive line is the problem, the torque converter would smooth out any erratic drive shaft rotation. The timing chain can cause some funny problems. One of my Chevy 350 timing chains wore out and actually jumped one cog. The engine ran but put out very low power. It sounded like a VW 1200cc air cooled engine. Luckily I was only 4 blocks from home when it went.

    Rockers sound like a good thing to look at. Maybe they are worn to the limit of what the lifters can accommodate but that doesn't explain how the hill can cause a miss fire. On the other hand if changing those parts solves the problem then you don't need to be able to explain the process that caused it.

    Bob K

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    Many thanks Bob, I appreciate all your suggestions I guess I'll have to look closer at what it takes to replace those rockers and if it will be worth it.

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    You're in Hawaii. Your oil never gets "cool" as in the bottom of the viscosity range.

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    Quote Originally Posted by CraigS View Post
    You're in Hawaii. Your oil never gets "cool" as in the bottom of the viscosity range.
    Yeah Craig... It amazes me that they recommend 5-20 weight oil, since it never gets that cold out here. But that's what they say, so I just go with it.

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    Jim, I'm wondering if it has anything to do with wheel speed sensors sending the computer readings (of what ever speed your doing going down the hill) and fuel being sent to the injectors not matching the vehicle speed. You said you coast down the hill but you would need gas pedal to go up that same hill. You would think the main computer would compensate for the fuel mixture versus wheel speed sensor data. Just a thought.

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    Quote Originally Posted by Phil V View Post
    Jim, I'm wondering if it has anything to do with wheel speed sensors sending the computer readings (of what ever speed your doing going down the hill) and fuel being sent to the injectors not matching the vehicle speed. You said you coast down the hill but you would need gas pedal to go up that same hill. You would think the main computer would compensate for the fuel mixture versus wheel speed sensor data. Just a thought.
    Thanks Phil, but I don't know what to make of it all..

    At one point, the light went out by itself. Then it came back on, and I cleared the code once or twice, and it never came back on again after that.

    I even checked for codes the other day, and there are no "pending" codes or anything.... The MIL is functioning properly according to the reader too.

    fwiw, I haven't been back to that same hill since my last post... That IS a really steep hill!

    I did replace the battery a couple weeks ago, so I wonder if that has anything to do with it.

    The only other thing I wonder about (aside from the valves), is if the #2 spark plug was loose at first. When I did the compression test, the #2 plug came out really easy. I think it was seated, but it was really easy to break it loose.

    There's still the issue of the low compression on #2 cylinder, and I did have to add more water to the radiator the other day, but it doesn't look like there's any water in the oil - as best I can tell anyway.

    Lately I noticed a new sound when rolling slowly down the road with my foot off the gas. Kinda like "puh puh puh" but it's very subtle. Seems like it might be once for each turn of the crank, but I dunno exactly. I'm assuming that it has something to do with that #2 cylinder compression, but it goes away when I hit the gas.

    I was looking into stories about the valve kit, and I saw one story about a guy who tried it but the problem was beyond what the kit could fix. Basically, he said it was a "round rod in an oval hole" kind of problem.

    So, for now, I'm just gonna drive it.
    Maybe I won't be doing any burnouts though

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    I've seen batteries do some weird stuff, but in this case, in my opinion, I think you might have a small head gasket leak. My wife's car has the same coolant issue. You can get test strips that test for exhaust gas in the coolant, but I can't remember where you get them.

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    Quote Originally Posted by 07cruiser View Post
    I've seen batteries do some weird stuff, but in this case, in my opinion, I think you might have a small head gasket leak. My wife's car has the same coolant issue. You can get test strips that test for exhaust gas in the coolant, but I can't remember where you get them.
    I'm afraid you might be right Cruiser... But I've been thinking that for years now...

    I have foam under the oil filler cap, and it's been like that for years, but the oil on the dipstick looks fine.

    A lot of people say that the foam on the filler cap is normal... "It's just condensation", and stuff like that.

    But when I look at that pic of the plugs in the post above, the color makes me think there might be something going on there between the #2 and #3 cylinders. They're right next to each other..

    I'll have to check with the local parts stores about that test kit, I wonder what I'll find in the radiator...

  15. #15
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    It's hard to tell what you'll find. Are you able to remove the head at all. Is that even an option?
    Go to amazon and search for this: http://www.blocktester.com/
    It could very well be condensation. You would know if it was a bad leak, there would be coolant in the oil or a whole bunch of white smoke.

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