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Northernmost Chevy Killer
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Discussion Starter · #1 ·
I purchased a 98 QC 1500 4x4 318 recently and it idles nice and steady. When I'm cruising around and let off the gas to slow up for traffic and then get back in the throttle it kind of hesitates and shakes. Now if I get in the throttle form a stop or hard into it from any cruising speed it does not do it. Only when lightly getting back into the throttle after letting the rpm's drop. It does it worse the slower I'm going(due to trying to get the big sucker moving again). The previous owner said he had a miss awhile back and that he changed the plugs and wires and it fixed it. I don't know how true that is. He also had it sitting for awhile so the gas might be no good. So I filled up with premium today and threw in a can of injector cleaner. When I got home I put it in park and noticed that it does it when ever applying the gas in park. Any ideas guys?Thanks in advance for any help.
 

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Northernmost Chevy Killer
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Discussion Starter · #3 ·
I've been doing some looking around and have seen this tps a few times. I have a haynes at the shop that I bought when I had my 99. I will look for where it is located tomorrow. Is this part expensive? I don't know if I have ever heard a vacume leak before. Is it pretty apparent? Would it help to have someone feather the throttle while I listen?
 

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Northernmost Chevy Killer
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Discussion Starter · #4 ·
I also have noticed(without even doing the math) that my gas mileage is in the crapper. I can just tell the needle is moving far too fast. Maybe that will help diagnose the problem.
 

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******* SPACESTATION
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Post #4
I also have noticed(without even doing the math) that my gas mileage is in the crapper. I can just tell the needle is moving far too fast. Maybe that will help diagnose the problem.
with the mileage being in the crapper check the vacuum hose for the map sensor. Map sensor is on the front of the throttle body, IAC is on the back, and TPS is on the driver side. If you have the factory air box it'll be easier to see with it removed.
Might also check the O2 sesor. If it doesn't hesistate at WOT (wide open throttle) it could be due to the fact that the O2 sensors don't read during WOT.
usually the 02 sensors will throw a code though.
 

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Northernmost Chevy Killer
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Discussion Starter · #6 ·
chizzle1 said:
with the mileage being in the crapper check the vacuum hose for the map sensor. Map sensor is on the front of the throttle body, IAC is on the back, and TPS is on the driver side. If you have the factory air box it'll be easier to see with it removed.
Might also check the O2 sesor. If it doesn't hesistate at WOT (wide open throttle) it could be due to the fact that the O2 sensors don't read during WOT.
usually the 02 sensors will throw a code though.

I will check those tomorrow. I thumbed through the haynes manual today and located all you mentioned above. I was able to test the tps with a multimeter and it tested out fine according to the book. It does not do it if I put my foot in it. It really shudders bad when I let off and let the rpm's drop and then get back in it in the 45-50 mph range. It really shakes like a leaf in that range for a few seconds until I really get in it. I'm hoping to figure this out without dumping a bunch of money into it. Thanks for your help guys. Let me know if you think of anything else.
 

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While my truck is a V10, it does have a TPS and MAP sensors. I had a similar issues with poor idle and I could stall the truck in park if I pushed the throttle down slowly. It took the replacement of the both sensors before the problem was fixed. I would agree though that the TPS would be the place to start. My TPS was about $80 from the dealer. The MAP was a pretty big hit at about $180 to $200.
 

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Just to follow up on the TPS thought-a multimeter won't show you "dropouts" that you can see on a scope. Multimeter is not sensitive enuf to give you visual representation that you can rely on. BTW, is the Check Engine Light CEL on? or does it come on when driving? And to your early question on vacuum leaks-they make a hissing sound, sucking air thru a crack in a line. With your engine running pull a vacuum line off of something and listen to it-that will get you going on vac leaks. You may set a code when you pull off a line-but we can deal with that. HTH
 

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Northernmost Chevy Killer
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Discussion Starter · #9 ·
I listened for vacuum leaks. None found that I can tell. I kinda figured that about the multimeter test on the TPS. It has not thrown a code at all. Except when I tested the tps. I cleared it though. I was hoping for someone to have had this exact problem and know what it is. After looking at the prices of some of these parts I have decided I can't just throw that kind of money at it trying things. Turns out I got a call today from one of my friends that was in Iraq. He is back and is working part time at the shop he worked at before he left. Same place I took my work van to. Anyhoo he's gonna let me bring it in after hours and look at the thing with the scanner so I don't have to blow money far parts that are good.The book did say that a scan tool was by far the best. Thanks for your help guys. I will let you know what happened.
 

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if you have a scanner tool you can read the reported throttle position from the TPS live with the scanner hooked up.
I would do this with the engine off for (obvious reasons). Try the full range from no throttle to WOT and see if there are any instances where the scanner shows any dropping / variation in the reported signal.
 

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Northernmost Chevy Killer
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Discussion Starter · #12 ·
Dodge318 said:
Don't overlook the EGR valve (probably plugged) with the poor mileage and hesistation. Mine did somthing very similar to that when my EGR went bad.

I will keep that in mind.How similar was your problem. Please explain. I have heard of cleaning these. Is that ok?If not do you remember how much you paid for the part?
 

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Mine did pretty much exactly what yours does. It would hesitate taking off and stumble letting off the gas but would cruise all day at 70mph just fine. It would also do it in park. I also had a catalytic Converter go bad and it also had similar symptoms. It's one of the two imho.
 
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