88 gmc pickup "loping"

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  #1  
Old 07-27-05, 02:26 PM
map686
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88 gmc pickup "loping"

Hey all, I've got a 88 gmc pickup, full sized that recently started to lope. By that I mean, it idles rough, but runs fine when I'm on the highway, but when I let off the gas it starts to run rough. It is not timing. My books suggest vapor canister, which I have replaced. There seems to be no vacuum leaks. So I'm thinking next guess would be the EGR valve or soleniod. Any thoughts how to test either or both? Thanks for any help!
P.S. are those $25 code readers any good.
Thanks, MP
 
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  #2  
Old 07-28-05, 03:25 AM
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you can get codes by jumpering the aldl connector pins a and b under the drivers side dash you just need a paperclip or a small piece of wire or something similiar to use for a jumper then turn the key on and count the check engine light flashes it will start with code 12 and will flash each code 3 times then will repeat if there is only a code 12 then there is no codes stored.
if you have any codes you can post the code here for more info.
lots of things could cause a rough idle but if it is loping or surging checking the egr valve to make sure its not stuck open at idle would be a starting point if the egr is closed at idle then you should check fuel pressure should be between 9- 13 psi low pressure could be caused by a weak pump or a regulator problem.
 
  #3  
Old 07-28-05, 06:19 AM
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Location: North Texas
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scan tool

I had a 89 Chevrolet with the OB1 system you can just jump the pins like said and count the flashes.However that scan tool does make it a lot more easy and comes with a book with all the codes.(I bought one).I now have the tool but lost the book but it does not matter since my cars now are all OB2.
Thats my 2 cents. To tell you the truth I did like the tool and used it several times on my truck to find the code. 25 dollars is a lot of money once you see that the tool is nothing more then a housing with the jumper wires built into it.
 
  #4  
Old 07-31-05, 08:18 AM
map686
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I am getting 43 code. Not sure which way to go now. either knock sensor,tps, esc, or est. I'm so confused.
 
  #5  
Old 07-31-05, 07:13 PM
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start by checking to make sure the knock sensor is still connected and plugged in it is located just in front of the starter and the connector commonly gets broken or may get disconnected.
is the check engine light on when the problem occurs?
if the light isnt on at idle or when the problem is occuring the code is probably not related to the problem you are having.
also might note that code 42 is often set when the timing connector is disconnected to check or set timing.
 
  #6  
Old 09-07-05, 12:17 PM
map686
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added all new emmisions parts, cat converter, plugs, wires, dist cap, rotor, timing in set correctly. any thoughts?
 
  #7  
Old 09-07-05, 12:37 PM
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if the check engine light is still on and you are still setting a code 43 you will need to fix that first again check the knock sensor, wiring, ecm, and esc module is about the only things that can cause a code 43 it will probably require a scanner to diagnose a code 43 they are commonly checked by tapping on the block with a hammer while reading the scan tool to see if knock is indicated which will require seeing a shop for diagnossis.
if you no longer have a code 43 or check engine light have you checked fuel pressure?
 
  #8  
Old 09-07-05, 01:23 PM
map686
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check engine light comes on sometimes. after I put some of these parts in and took the truck out and made it work, (hammer down and pushed it tryied to set a code) I got codes 22, 34, and 43. I'm wondering if the ecm is shorted or shot?
 
  #9  
Old 09-07-05, 07:06 PM
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Join Date: Feb 2005
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I've had a number of issues with these older trucks involving a bad ground strap. This can cause a number of issues. Just for kicks, run a ground strap from the engine block to the frame. I always do this on the passanger side of the engine compartment for ease. For what it would cost you in supplies, it's worth the shot.
 
  #10  
Old 09-08-05, 03:02 AM
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disconnect the battery for a few seconds to clear all the codes in the computer then after driving the vehicle and you have noticed the check engine light has came on recheck the codes and go from there, more than likely some of those codes was set while you were working on the vehicle or changing other parts I suspect it will only have one code stored in the ecm memory after clearing the others.
 
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