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chevy 1500 surging in overdrive

This is a discussion on chevy 1500 surging in overdrive within the Automotive Support forums, part of the Tech Support Forum category. I have a 94 chevy 1500 5.7l throttle body fuel injection. The problem is that when I'm in overdrive or


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Old 03-21-2011, 01:43 PM   #1
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I have a 94 chevy 1500 5.7l throttle body fuel injection. The problem is that when I'm in overdrive or low gear on the highway the engine rpm surges up and down usually between 1500 and 1600 rpm while in overdrive. After about 10 min of this the service engine light comes on and throws an o2 sensor. Any ideas? I've replaced the injectors, fuel pressure regulator, pcv valve, idle air control, tps sensor, o2 sensor and a list of more stuff... I also have a tbi spacer idk if that could be doing it?
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Old 03-21-2011, 02:15 PM   #2
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Is there vacuum going to the MAP sensor ?
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Old 03-22-2011, 12:19 PM   #3
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Yea the vacum seems fine. But I did make a mistake, I got my friends scanner last night after I got the check engine light to come on and the codes were dtc 32 and 42. 32 is egr and 42 is timing spark? I think I need to replace the egr but would a bad egr cause the other code to come up?
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Old 03-23-2011, 05:00 AM   #4
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Yes it would , If the EGR is leaking it would throw off the timing.
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Old 03-23-2011, 01:11 PM   #5
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Alright thanks so much I'm pretty sure its just a bad egr valve considering its the original with 135000 miles. I post back when I replace it and see if there is any difference
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Old 03-25-2011, 06:57 PM   #6
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I replaced the egr valve, it still does it but now the codes are 32 33 and 42. 33 is the manifold absolute pressure high which means the vaccum is low. What would cause this?
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Old 03-25-2011, 07:22 PM   #7
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Vacuum low pressure is when there is a restriction or complete blockage in the amount of vacuum going to MAP the sensor. The problem you have to contend with is the code 42 which the computer has detected no spark advance. The problem can come from the ignition module or the computer.
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Old 03-26-2011, 06:35 AM   #8
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Ok so is the 42 code setting off the other 2 codes? And where would I begin to solve this? I just timed the truck the other week and there is a timing connector wire you need to remove before timing. After I timed it I plugged the wire back in. Should I have disconnected the battery after that?
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Old 03-26-2011, 06:59 AM   #9
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Its very possible, you should erase the codes and see if the CEL comes on again.
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Old 03-26-2011, 04:43 PM   #10
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Well tryed resetting it and that didn't do anything. Is the 42 code causing the other two codes or vise versa? And if so where should I start to try and fix it?
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Old 03-26-2011, 05:49 PM   #11
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Start by taking your DVOM and test open to ground the advance wire from the ignition module to the computer. If the wire tests OK then the ignition module has to be tested for the pulse signal to be present and to do that you need an oscilloscope or labscope. The old fashioned method is replacing the ignition module first, if it doesn't solve the problem the computer gets ditched as well.
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Old 03-28-2011, 01:39 PM   #12
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Could it just be a bad map sensor? I have a chilton book and it says that with the key on the sensor should read 5 volts. Mine read about 3.2 volts. Wouldn't that mess with the ignition timing?
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Old 03-28-2011, 04:42 PM   #13
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Yes it would, if no vacuum goes to the sensor or the diaphragm is messed up in some way it can set off timing codes because the computer adjusts air/fuel by what the sensor tells it. By probing the green wire to positive on the meter and the black wire to body ground with KOEO the sensor should read 5v. The MAP sensor readings should begin to drop when vacuum is applied, for every 5Hg of vacuum the sensor should drop 1 volt. The sensor should also be able to hold vacuum if it doesn't its gone, remember sensor voltage changes as engine load changes.
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Old 03-29-2011, 01:39 PM   #14
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Ok thanks I'm going to test that and the egr solonoid. I'm not sure if I did not reset the computer correctly but I left my battery disconnected over night and now the only code is 32 which is the exhaust gas recirculation. Considering it is a brand new egr valve, the only other thing I can think of is the egr solonoid
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Old 03-29-2011, 02:42 PM   #15
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Is your EGR vacuum actuated or electronic ?
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Old 03-29-2011, 03:20 PM   #16
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Vaccum
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Old 03-29-2011, 03:20 PM   #17
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Atleast I think so? Its got one vaccum line and that's it
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Old 03-30-2011, 03:39 AM   #18
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Trace the vacuum line, does it go to a solenoid or to an intake port ? If you have a hard time following the vacuum line look at your sticker under the hood that has the vacuum diagram for your engine is there an EGR solenoid ?
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Old 03-30-2011, 02:25 PM   #19
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Okay last night I went over to my mechanics house and we were checking some stuff out. Yes there is a egr solonoid. We first took a vaccum line straight from the throttle body to the egr valve will little difference in idle. He told me that it should stall or almost when the the solonoid is bypassed. He figured the ports in the manifold where the egr valve bolts up must be clogged with carbon. Next we tested the solonoid by starting the truck then taking the two prong wire harness and grounded one and running another wire to the positive with no difference. We came to the conclusion that the solonoid is shot because when shorted out it does not do anything so I will be replacing that tonight and posting back my results. Thanks for your help I can normally figure most of this stuff out but I needed someone on hand to help as well.
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Old 04-01-2011, 01:23 PM   #20
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Well I replaced the solonoid and it is still doing it...
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