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HELP! Service Engine Soon Light - 2001 Express

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  • HELP! Service Engine Soon Light - 2001 Express

    I have a 2001 Express 3500 5.7 liter with a factory Impco CNG conversion. So far I've replace the HPR, the distributor, and some O2 sensors.

    Now I have a SES that's been on for several months. I've been chasing it without success. I took it to a friend who's a mechanic and has a Snap-On reader that checks all of the sensors. The history showed 3 MAF sensor faults, one catalytic convertor fault, and one no data fault. He reset the computer 5 times, and each time the SES comes on as soon as the engine is started. He thinks it's the PCM itself that's at fault, because the PCM wasn't showing any fault codes, but the SES was on anyway. And, of course, it won't pass emissions with the SES on.

    Here's my concern: I'm not sure how the AFCU interfaces with the PCM, and I'm not sure replacing one or the other will fix my problem. Any ideas or experience would be appreciated.

  • #2
    Re: HELP! Service Engine Soon Light - 2001 Express

    If the van is out of CNG (running on gasoline) does the SES come on right away? If the underhood CNG fuse is pulled and codes reset does the SES come on right away? The problem with the reader you are using is that it cannot read the unique codes or data from the AF ECU. Therefore you might be seeing codes that are a result of a problem and missing the code that code directs you to the actual problem. The only reader that has the required PID's for reading the AF ECU is the GM Tech2. Find someone with a Tech2 and scan the vehicle again and send a list of the reported codes. I will look them up and go from there. From the list of replaced parts, it looks like you are bouncing around chasing this. Why did you replace the distributor--gear wear? How did you adjust the position of the distributor when you replaced it? This must be done with a scan tool. The SES comes on for rather minor issues unlike the check engine (MIL) light. Something as simple as an oil life monitor will set the SES.

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    • #3
      Re: HELP! Service Engine Soon Light - 2001 Express

      Originally posted by siai47 View Post
      If the van is out of CNG (running on gasoline) does the SES come on right away? If the underhood CNG fuse is pulled and codes reset does the SES come on right away? The problem with the reader you are using is that it cannot read the unique codes or data from the AF ECU. Therefore you might be seeing codes that are a result of a problem and missing the code that code directs you to the actual problem. The only reader that has the required PID's for reading the AF ECU is the GM Tech2. Find someone with a Tech2 and scan the vehicle again and send a list of the reported codes. I will look them up and go from there. From the list of replaced parts, it looks like you are bouncing around chasing this. Why did you replace the distributor--gear wear? How did you adjust the position of the distributor when you replaced it? This must be done with a scan tool. The SES comes on for rather minor issues unlike the check engine (MIL) light. Something as simple as an oil life monitor will set the SES.
      The SES comes on on CNG or gasoline (fuse pulled) as soon as the engine is started. The replaced parts actually came before the SES problem. The first thing to go was the HPR, then the distibutor, then some rear wheel seals (and of course, the brakes) - I was just listing my frustrations with the van. The SES actually came on about three months after I replaced the distributor, and it's been constant since then. The only exception is during a recent trip through Wyoming. My gas gauge dropped off quickly, I pulled off and filled both the CNG and gasoline, and the SES went off for about 120 miles. When it was restarted the SES was back on.

      The SES on the Chevy van is the MIL - it's just the terminology Chevy chose to use. It's not the same as the Service Needed light on some cars.

      Who might have the GM Tech2? Would it be a dealer-only item?

      Thanks.

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      • #4
        Re: HELP! Service Engine Soon Light - 2001 Express

        Yep, you are right--the van uses the SES light as the MIL. Never had a SES on my van so I assumed There was either both or only a MIL. Anyway, the SES light signal comes from the PCM (gasoline), therefore with the AF ECU inactivated by removing the power fuse it isn't possible to send the MIL request to the PCM unless you have a grounded wire coming from the AF ECU or there is an internal short in the AF ECU. You could remove the two connectors going to the AF ECU and see if that makes the problem go away. Did you re-check the distributor adjustment with a scan tool? If it is off, the SES will be set. The snap-on reader should be able to read the cam sensor to distributor relationship. When the test is active, it will read in degrees of retard or advance in the relationship and needs to be adjusted to zero. After being adjusted to zero, if it moves from zero it's a good early warning the the distributor gear is being eaten by the cam gear. My van was a very low mileage van (17K) when I got it and it already had a distributor replacement under warranty because of the gear. All GM dealers have the Tech2 and many local shops have them. As your problem is not specific to the CNG system, most good readers will diagnose the problem. I own the same model and year van that you have but haven't worked on it much with the exception of cap and rotor stuff, pulling the distributor to inspect the cam gear and replacing rotted vacuum hoses. I also had to drop the tanks to replace the rubber tank isolators which had gotten to the point of having the tank brackets starting to contact the tank overwrap right were the two clamp halves meet. That's a bad thing and something to keep an eye on. BTW, when you scan with the SES light on, what codes are being reported and are they always the same? Can you get a "readyness" indication on the scan tool with the light on?

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        • #5
          Re: HELP! Service Engine Soon Light - 2001 Express

          That's the weird thing; it's had some random codes, but it's not consistent. When I pulled the codes with a reader from O'Reilly's I got codes for O2 sensors (I also replaced one of those - $75), but when he pulled the codes with the Snap On reader, there were no O2 sensor codes, just the ones I mentioned. But when he cleared the codes, the SES came back on immediately when it was started, even though it wasn't showing any codes. He reset five times; no codes, but the SES is still on.

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          • #6
            Re: HELP! Service Engine Soon Light - 2001 Express

            We're back to the question of were the MIL request is coming from. The PCM can set it itself during gasoline operation or there is a connection from the AF ECU to the PCM that can also request a MIL. If that line is grounded either by a short to ground along the wire or by a internal problem in the AF ECU then the MIL will come on. The readers you are using cannot read DTC's coming from the AF ECU. However, with power removed (CNG fuse pulled) the AF ECU cannont send the MIL request unless internally shorted. Removing the Blue and White connectors at the AF ECU and checking the harness back to the PCM for damage would eliminate the possiblilty that the MIL request is coming from the alternative fuel system. If the harness checks out and with it disconnected you still see a MIL, then the PCM is the problem. If the MIL is out and you plug the connectors back into the AF ECU (without the CNG fuse installed) and the light comes on, then the AF ECU is the problem. If everything else seems OK--the van runs normally on both gasoline and CNG--and the MIL only comes on if you plug in the AF ECU, then because of the cost of the AF ECU replacement, then I would cut the wire from the AF ECU at the PCM that is sending the MIL request. If you get to that point I will get you the wire color and connector pin location at the PCM.

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            • #7
              Re: HELP! Service Engine Soon Light - 2001 Express

              Sorry to jump in your discussion here but I happened to catch the fact that you replaced the distributor. My 2002 Express 3500 Van (5.7L, bi-fuel) was setting the Check Engine (SES/MIL) message quite often a few years back. Codes were mostly related to cam offset and misfires I think (might have been others as well but I can't remember). It was often worse in wet weather and at one point the vehicle would die for no apparent reason (usually on a rainy day). I trouble-shoot it to the distributor and replaced the cap and rotor and it would go away for a while (~ 6months). After about 6 months it would return again and I would go through the same thing again. I tried a rebuilt distributor and it lastest about 6 months and started the same thing again. I finally replaced the distributor with an after-market performance distributor and have not had the problem since. I believe the factory distributor has moisture infiltration issues. Need to make sure you get the distributor aligned corectly or that will cause all kinds of problems as well. Hope this is of some value to you.

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