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Truck accelerates on its own


Rgbraun1

Question

1999 Silverado 5.3 vin T.

Hello, I'm new to the forum. Here's my issue with my truck: I felt the slightest surge while cruising...I planned to do my ujoints so while replacing the U-joints and carrier bearing I decided to clean the throttle body and ended up replacing the IAC. On test drive I got a violent shake so I pulled the driveshaft and 180 degree turned the rear shaft on the slip yoke. The shudder went away but i noticed the the truck accelerating on its own, i put in gear and tap the gas and can take my foot off the gas and it will slowly accelerate through all the gears and gain speed. I put on a new TPS. Drove the truck 200 miles home (did all this work at my dads garage) and got a shake bewtween 35-45 mph and the accelerates on its own. Started getting different codes so I replaced EGR, MAP, MAF, throttle body gasket, engine temp sensor, plugs and wires, PCV and PCV grommet. I also replaced the connectors for the TPS and IAC. I finally took it to dealership and it took them 2 days and 3 techs to tell me it was intake manifold gaskets. So I replaced the gaskets (and the knock sensors and harness) but it still accelerates on its own. So, as it sits. My truck still accelerates on its own and it shakes at 35-45 mph. Is the shake from an imbalanced driveshaft? Or a bad torque converter? Can a bad torque converter cause the accelerates on its own problem. Also, while cruising, I can take my foot off the gas and the truck will maintain its speed. I have tried removing the cruise control fuse to be sure it wasn't faulty. Hoping someone can help. Thanks.

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I feel totally confident in my ability. I took it to the dealership because I'd been chasing the "accelerates on its own" issue for a few months. So, I replaced the IAC and TPS when I cleaned the throttle body. Because it was accelerating on its own. So posting on other forum and searching the net....it was suggested the coolant temp sensor was not sensing temp causing CPU to up the idle thus causing the truck to accelerate at will. Replaced that, didn't help, moved on to the o2 sensors etc. I do feel like I was throwing parts at it but followed different suggestions. Took it to the dealership hoping they would hook it up to their scanner and be able to read live data and give me a professional opinion. They kept it all day and claimed their top guy couldn't figure it out So they kept it another day for a second opinion and they claimed it just had an intake manifold leak and wanted $565 . So I took it home and did it...and did new knock sensors and harness while I had it off. I've had this 99 truck since i bought it brand new and in 2 years plan to pass the keys to my son. So putting new parts in didn't bother me too much. I'm just frustrated that I can't figure out the accelerates on its own thing. I think the driveshaft is out of balance and that's why I get the shudders and shakes from 35-45 mph. I've heard that after throttle body cleanings and new parts you should have a Tech 2 idle relearn done.

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Heavy5.7,

I tried the carb cleaner/vacuum leak test but couldn't find anything. Can't get to the "inside" edge if the intake manifold to spray it though. The Chevy tech guy at the dealership said it was the intake manifold gasket. I replaced it myself with the new heavy duty Fel-Pro. Throttle linkage had no play, throttle cable/ cruise cable seem fine. I tried lifting the gas pedal as it accelerates on its own and it didn't do anything. I've pulled the cruise control fuse to make sure that wasn't it. I've put about 200 miles on it since all the work and it accelerated on its own out of the driveway....and hasn't really done it since. It is holding speed/ not decelerating as quickly as it used to.

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If I read this correctly, the truck ran fine prior to the throttle body cleaning and IAC replacement? Try putting the old IAC, that I assume worked, back on and see what happens. If your truck didn't accelerate on its own prior to TB cleaning and changing the IAC, maybe you got a junk IAC. Alot of junk chinese parts out there from AZ and Advance Auto. Or maybe you kinked the throttle cable or don't have everything sealed up good causing a situation of sucking unmetered air into the engine and the computer trying to compensate for it.

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I hear the same dealership experiences from everyone I talk to, whether in person or over the internet. In light of this, I will confidently say that you're MUCH better off bringing a vehicle that is past it's warranty period to a local garage. They have access to any tool needed, even a ECU reflash - this is NOT a dealer only service! The rub is, you've got to weed out the chaff.

 

 

GM is in the business of making money producing & selling automobiles, & selling maintenance on said autos - they gain nothing by making your vehicle last forever. I dare say they would like it more if it didn't - that way it forces more people through the showroom doors as the used vehicle surplus dries up sooner than it should.

 

If you want that problem fixed, find a competent local mechanic to do so.

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Because it was accelerating on its own.

$20 will get you a cheap scanner that will interface with your phone.

When the truck is started and idling just sitting in park does the idle speed increase "on it's own"?

Did you compare the old & new IAC to insure the pintles were the same shape dimension and length?

Everything was OK before the repairs?

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No stored codes. The acceleration problem seems to have diminished since i replaced the intake manifold gasket. It's still there, but not as bad as it was. I replaced the PCV grommet. I'm wondering if the grommet and valve at the brake booster is leaking. Also, wondering about the fuel purge valve solenoid. It sits on top of the intake manifold and sits up a little high. I can see it taking some unintended abuse as I worked on the TB originally and now especially since I had the intake manifold off and sitting upside down as I cleaned it.

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The best mechanics, I'm sorry but they do not work at dealerships.

 

Luckily my friend is a good one so when I have issues I just call him.

 

Check the vacuum lines they are always a big source of surging it's the only thing you haven't mentioned checking.

 

Also do not rule out the ECM is going bad. Average Capacitors dry out after 10 years. So they start getting weak.

 

The fuel pumps are no better at the age your truck is as well.

 

I bought a car once very cheap that would just start and stall but he said it was surging before it died. I bought it and knew all it probably need was a fuel pump.

 

Replaced the pump it still ran ruff changed out the ECM it ran like a clock and 1 put another 250,000 on it no issues, it had 150,000 on it when I bought it.

 

So $600 for a broke Cutlass another $700 for the parts so it cost .0.0052 cents per mile good return ha ha.

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My '86 Grand Marquis is still on it's ORIGINAL fuel pump! We bought that car 14 years ago this week. 239k miles. She's hibernating for the winter in the barn - that's how it's survived 30 winters. :)

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My '86 Grand Marquis is still on it's ORIGINAL fuel pump! We bought that car 14 years ago this week. 239k miles. She's hibernating for the winter in the barn - that's how it's survived 30 winters. :)

 

I've seen allot of vehicles make it for years to, but I've seen many more not ha ha...

 

I looked at a 2002 Cadillac STS stalling when it goes into gear a few weeks ago. I decided not to get it told the owner I think it just needs a fuel pump. Three days later he texted me a thanks and his add is pulled. ha ha. Could have had a very clean on for only $800.

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Wow, pretty cheap!

 

Thing with the OBD-II ones is, you never know what gremlins are going to pop up sooner or later. May be a good thing you let it go! The '95 and earlier models, for me, are much less of a headache.

 

Yeah, I've sent my fair share to the scrapyard over the years, lol. My '85 Monte was one of them. Hit everything but the lottery with that one. :lol:

 

85Monte.jpg

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I also replaced both upstream o2 sensors. And for clarification...the truck accelerates on its own at various times ....i.e. Come, warmed up....maybe won't do it all day and then does it the next day. Same with the "maintaining speed on its own.

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