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Thread: Running poorly/misfiring, code 33

  1. #1
    Fuel Injected! JeepsAndGuns's Avatar
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    Running poorly/misfiring, code 33

    Long time member, but I have not been on here in years because everything has been running perfectly.
    Well that has changed and I need some help figuring out this problem.

    AMC/Jeep 401 V8. 16197427 ecm running $0D. Started out as TBI but later changed to MPFI by swapping out he resistors in the ecu and doing the needed programming changes in the BIN.

    So everything has been great for years, running great, very reliable, super fun.

    So randomly a month or two ago, I noticed what sounded like it was dropping a cylinder or two at cold start. Sometimes it lasted a few seconds, sometimes you had to start driving for a few seocnds then it suddenly cleared up and ran fine.
    One day it started doing it when I was about halfway into town and did it the whole way back home. I did not have time to check anything at the time.
    The next weekend I look everything over, all the spark plug wires looked good. Distributor cap and rotor look good. I cranked it up and it ran fine.
    It did this off and on a few times, then on the 4th of july, I went to drive it to lunch, and it was missfiring hard. You could hear it cut in and out. Like whatever the issue was, was coming and going intermittently.
    I messed around with it and unplugged each injector 1 at a time seeing if I could narrow it down to one cylinder. I could not.
    I checked the spark on a couple cylinders and it seemed good.
    The issue/misfiring seems ignition related, as the AFR on the wideband still looks good. Not running rich or lean.

    I am running a HEI distributor I modded several years ago and running the 7 pin module.
    I had a spare coil, so I figured I would swap it out, just in case it was breaking up. No change.

    When I say misfiring, it is not backfiring or popping, but just sounds like it is not running on all cylinders and has no power.
    I tried running it after swapping the coil and it is still doing its misfiring thing, but then I just randomly look down and saw the tach jump.
    So I thought, heck, I bet I have a bad pickup coil. I swear I want to think I saw the tach jump, when it cleared up for a second.

    So this past weekend, I pull the distributor, swap out the pickup with a spare I have and reassemble. I marked the location of the distributor and rotor and get it back in as close as I could to where it came out, so I could get it going enough to reset the base timing.
    Well, still no dice. It actually runs worse now. It will barely stay running, and now wants to pop out the exhaust a little.
    I'm sure the timing might be off a degree or two, but it should not be enough to cause this issue.
    I go ahead and swap out the 7 pin module with another spare I had just for poops and laughs with no change.

    I finally break down, dig through my storage and find my autoprom. I fire up the laptop and hook it up.
    It is showing error 33, MAP high.
    I think, dang, was that my whole problem all along, a bad map sensor?
    I swap it out for a spare I have with no change. I disconnect the battery to reset the ecm with no luck.

    So I turn the key on, connect to the ecu (engine not running), hook up a hand operated vac pump and apply vac to the map sensor. I get no reading, it just shows map 100.
    I try both sensors with no readings from either one. The spare I know was good when I put it in storage.
    I check the connector and I have the 5v refrence, and have ground.
    I unhook the ecu and check the signal wire, thinking it is broke. Nope, I have continuity and it ohms out with no resistance.
    However, I can get no map reading on the datastream. I have bench tested several ecms in the past and using a vac pump on the map sensor, always showed it on the datastream. But it will not show anything now.

    When I start the engine, the map will read, but it stays at map 80-90. So naturally, the engine runs like pure crap, cause it thinks the map is that high while at idle, and the fuel/spark is being adjusted to try and account for that.

    Last ditch effort, I break out my spare ecu I have (yea, I like having spare parts). Same thing with the other ecu.

    So I am pretty much at a loss and at my wits end. Why, if I am getting power and ground, and the signal wire seems fine, am I not getting a map sensor reading in the ecu/datastream. But with the engine running, I get extremely high map reading, but only when the engine is running.
    I even tried hooking up the vac pump, then starting the engine, and it still reads high map (and runs like crap).

    Just looking for any help or suggestions.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

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    Fuel Injected! JeepsAndGuns's Avatar
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    I can pick up a vac gauge and hook it up this weekend.

    However, any idea why I can not get any map sensor reading on the datastream?
    I have bench tested many ecu's in the past and used a hand operated vac pump on the map sensor and saw it on the datastream.
    I am just confused as to why I am not seeing it now.
    It is acting like either a broke wire between the sensor and the ecu, or it is not getting power. However I have tested both and they both appear to be good.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  4. #4
    Fuel Injected!
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    Well first I'd do a sanity check. While the car is running and you're connected to the ECM and datalogging, unplug the MAP sensor.

    What happens?
    1990 Corvette (Manual)
    1994 Corvette (Automatic)
    1995 Corvette (Manual)

  5. #5
    Fuel Injected! JeepsAndGuns's Avatar
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    It will not stay running on its own right now. I have to keep it running by feathering the throttle. If I let off, it stalls within a second or two. Not enough time to get to the sensor and unplug it.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

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  7. #7
    Fuel Injected! JeepsAndGuns's Avatar
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    That was one of the early things I did with no change.
    I also tried another spare ecm I had with no change.

    When I pick up a vac gauge this weekend, I'm gonna try and get some back probes and test the map sensor pins with the sensor plugged in.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

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  9. #9
    Fuel Injected! JeepsAndGuns's Avatar
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    Well the parts store did not have a vac gauge or back probes.
    However I feel like the issue is electrical and not mechanical, seeing as how it was intermittent at first. I may end up ordering a vac gauge online just to have one.

    I did raid my wife's sewing kit for a needle and used that as a back probe.
    I ended up not having anywhere near as much time this weekend as I thought I was going to have.
    With the sensor plugged up and the ignition on (engine not running), I have 5v at the power wire, and have 4.88v on the sensor out. The voltage drops as I apply vac to the sensor.
    I then go to the ecu and check the pin there. I have the same voltage readings (ignition on, engine off).
    So it would appear it is making it to the ecu. However I did not have the laptop hooked up.
    I took loose the distributor and gave it about one degree of a turn just to see what would happen.
    I got it to fire off and run. Still ran like crap and missfired like crazy, but after about 30-60 seconds, I got it to idle on its own (still running poorly).
    While it was running, I was feeling around the wiring and just looking things over and it suddenly stumbled badly and died.

    I am starting to think I have a bad wire, that is somehow testing good, but for some reason is not getting the signal to the ecu. Or is showing good when I test it, but is not when the engine is running (for some reason)

    I have some spare pigtails and terminals for the weatherpack connector for the map sensor, but I do not have any spare pins or pigtails for the ecu connectors. I do not really want to cut the wire close to the ecu to run a new wire, only to find it is that that wire. I need to find some pins and make a temporary new wire from the map signal to the ecu and test it.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  10. #10
    Super Moderator dave w's Avatar
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    Maybe this connector kit from Casper Electronics will be helpful? http://www.casperselectronics.com/ca...product_id=552

    The kit has MAP pins and TBI ECM / PCM Pins
    Last edited by dave w; 07-24-2023 at 04:37 AM.

  11. #11
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    Hey, nice to see you here.

    MAP sensor high indicates the ecm is reading high voltage on the MAP signal line during specific operating conditions. For default configuration, here's the code definition:
    33 MAP sensor signal voltage was too high (low vacuum) for 1 second when throttle opening was under 20% and engine speed was above 1250 RPM.

    You should be able to see MAP voltage on the scantool in key on / engine off condition to compare the reading obtained at the PCM wiring harness connector using a DVOM. Additionally, you can create simple resistor networks to connect between 5V, signal, and ground to create a "between" voltage to send to the pcm.

    In a stock wiring harness the 5V circuit and GND circuit for MAP are shared with the EGR valve. Any issue which can cause a short to 12V on GND or 5V REF or an open on the ground circuit will also raise voltage on the signal line as seen by the ECM. To catch this on the DVOM you should be measuring voltage between the sensor ground and the sensor signal line.

    Since the engine reacts badly while wiggling the harness, I would likely try connecting the laptop to watch ALDL data then taking hold of the harness and performing a Ford "Wiggle Test." Watch for significant changes in MAP or another sensor reading on the laptop.

    I have had troubles years ago with intermittent code 33 on TBI equipped 1995 vans. In one case I found a broken wire in the harness causing an intermittent open. It was only apparent when the engine was very warm. In another case an aftermarket EGR valve caused an issue. In both cases it was frustrating to find because there were no symptoms showing while the van was in the shop.

    Keep us updated!

  12. #12
    Fuel Injected! JeepsAndGuns's Avatar
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    Thanks for the tips.
    The jeep does not have a EGR and all the related wiring was removed from the harness long before I installed it years ago.

    I forgot to include it in my original post, but one of the times I had the laptop hooked up, I applied vac to the map sensor and had my wife watch the screen while I moved and wiggled the harness under the hood. She said it never moved.

    I have had wires do strange things in the equipment I work on over the years (I am a small engine mechanic). Lots of intermittent issues and they are always the hardest to trace down.

    My local junkyard is about a hour away, but I have a gun show to go to in a couple weeks in the same city it is in, so I plan on stopping by there and picking up a pigtail for a ecu so I can have some spare pins.
    I will then make and run a new temporary wire from the map to the ecu and see what happens.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  13. #13
    Fuel Injected! tplasek's Avatar
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    You can find the TBI ECU pins on Ebay. I bought a pack of 15 for around $10.

    Someone also previously mentioned that you need to use the sensor ground when measuring the voltage on signal or sensor voltage. This is important as this is the voltage the ECU sees. DO no use a chassis ground.
    Last edited by tplasek; 08-03-2023 at 07:31 PM.

  14. #14
    Fuel Injected! JeepsAndGuns's Avatar
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    So I got a ecm pigtail at the junkyard and finally had some time to try and fool with it today. I have just been super busy lately.

    I made me a new wire for the MAP signal. I just laid the new wire on top of the harness as it was just temporary.

    I still could not get any reading on the datastream of the map sensor with the engine not running. Everything else seemed to read ok.
    I started it and it still ran like crap, I had to keep it running with the throttle.
    I did get map readings with the engine running. About 70-80 at idle, and it would go up if I opened the throttle.
    After keeping it running for a few min, it would idle on it's own, but barely, and still ran like crap. Then after idle for a short bit, it would stall.
    I had to run into town to do a couple things and finally found a vac gauge at the parts store.
    I disconnect the vac line going to the charcoal canister purge and hook it up there.
    At idle, it is around 5, maybe a little more in-hg. When I open the throttle a little, and the datastream shows 50 kpa, the vac gauge read about 15 in-hg.
    Vac seems to go up when I open the throttle, then drops down at idle.
    However, this engine has never really had high vac at idle.
    This is the camshaft I have in it: https://www.compcams.com/xtreme-ener...c-290-401.html
    I figure since it was made for carbs, and having 110 LSA is probably the reason for the generally low vac at idle.
    However it has never been a issue before and it has been great and trouble free for years after I tuned it.

    I will attach two log files.
    One was from several years ago. I have no idea what I was doing, but it appears I made a short trip down the road and back. You can see what everything read while it was running normally.
    Then I will attach the file I recorded this afternoon.
    I started the recording, waited a couple seconds then started it. You can see from the tps readings, I was having to keep the throttle open to keep it running. I did let it idle for a little bit and it roughly did, then I opened the throttle some for a bit then let it idle again. It did, but still poorly, and then it stalled. I waited a couple seconds then stopped recording.
    It looked like the map went real high as it shut off. I do not know if it was just the result of the engine stalling, or it was the reason the engine stalled. I tried watching it a couple times but do not see one thing standing out at me.

    The files are the dates I made them.
    $0D, 16197427 ecm converted to MPFI.
    Jeep/AMC 401

    I removed the new wire and hooked the old one back up and it did not really seem to change anything.
    I have also cut all the zip ties and pulled off almost all the wire loom to inspect the wiring and have yet to find any spots with issues.
    I double checked I was getting 5v to the map sensor, ground, and when using a hand pump, map sensor voltage lowers as vac goes up.
    Attached Files Attached Files
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

  15. #15
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    It sounds like the MAP readings when the engine is running are pretty close to accurate. I think I would move away from trying to get MAP readings to display on the laptop during KOEO and move toward diagnosing the driveability issue. MAP codes can be set by a very poorly running engine.

    Assuming the spark is working and the timing is close, I believe the next test I would make is to ensure all eight injectors are firing. Injectors not delivering fuel can seem very much like a spark issue. I would probably start with a noid light at each injector connector, cranking the engine and watching for the light to blink consistently. Then I might do my best to get the engine to start and I would use a screwdriver to "listen" to each injector click or use a special tool that indicates when an injector is clicking to try to determine if there's a problem with injectors.

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