2008 impala LT 3.9l engine tick and cuts out under acceleration - Chevy Impala Forums
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post #1 of (permalink) Old 04-16-2019 Thread Starter
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2008 impala LT 3.9l engine tick and cuts out under acceleration

Hello. 20 year mechanic here but I worked on busses and diesel engines mostly. I fix cars but some of the more complex issues I dont have alot of experience with.

I've got a 2008 Chevy Impala LT 3.9l. I've been hearing a ping from the engine that sounds like pre ignition to me for quite some time but could never verify it because it's hard to hear while driving and it didnt change with higher octane fuel pr e85. Then it started to cut out when trying to accelerate at highest speeds. If you let off the accelerator and hit it again it will cut back in and accelerate. Almost feels like a tps issue, however I replaced the pedal assy and the tps on the throttle body a year or 2 ago due to active trouble codes and the reduced engine power on the dic. I did get some trouble codes before this started happening related to the pedal and throttle body with the reduced engine power light on, so I cleared it to drive and it didnt throw the code again. I pulled put the throttle body and verified it opens and closes appropriately with the pedal. I tried to monitor the tps commanded tac and pedal positions on my scan tool but I'm not sure how far off the numbers should be. The commanded and tps seem to match but the other positions values are are different. It doesnt throw a code so I assume they are valid.

The other thing is the tick in the engine. At an idle you can clearly hear it. I removed the belt to verify it's in the engine and it is. Same noise with belt off. Louder than a lifter tickbut not a knock like a rod.

My thoughts are fuel pressure starving the injector maybe? Would that cause a injector to make that noise? No misfires or hard starts. Just no power when at highway speeds and pedal is pushed down to accelerate. Falls on its face like a tps issue but no code and cant see anything goofy on the live data, but not sure what exactly it should look like. I do not have a fuel pressure tester, but if someone tells me I need to check it in could rent one.

222k miles on this impala. Afm works still. Or dod, whatever it is called. Cleaned the maf... plugs and wires are new.well maintained by myself since I bought it at 100k. New air filter. Original fuel pump as far as I know.

Any gm techs out there seen this before? Or anybody else have any clue where to start? I dont have alot of money so I cant afford to throw parts at it. I'm in school for IT now because I never liked being a mechanic. Any help is greatly appreciated.
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post #2 of (permalink) Old 04-17-2019
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Intermittent codes for the fly by wire throttle.

Have you checked the wiring at the throttle body and the engine grounds?

Have you looked over the accelerator pedal itself. IIRC there was a batch with solder joints that cracked. Probably low lead solder issues. Lead is good. Tin is more brittle than lead. I've re-worked several problem circuit boards in vehicles using 60:40 Sn:Pb for the repairs and the problems ceased.
SPECIAL COVERAGE: Accelerator Pedal Position Sensor - 2006-2013 General Discussion - NAIOA

Intermittent ticking on a 2007/08 3.9L could be AFM lifter problems but I'd also look into the bypass rating of the oil filter.

Have you logged data on the highway to observe what the fuel trims, O2, TPS, APP, and MAF are doing?
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post #3 of (permalink) Old 04-17-2019 Thread Starter
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Thanks for the reply man. Yeah I know all about solder. It's a hobby of mine. Was that batch you are talking about acdelco replacement ones? Because I did replace it with a acdelco one from car quest.

Yeah the tick could be a lifter from the afm i suppose. But wouldnt I get a miss or feel something weird when it engages or disengages? As far as the oil filter it's a delco one that i always run.

I dont think my scan tool will monitor fuel trims. I can see tps maf and o2. Not sure what app is. Thanks for your help man.
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post #4 of (permalink) Old 04-17-2019 Thread Starter
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Also the tick sounds just like pre ignition . Do you think it's possible the timing jumped a tooth and that is causing detonation and performance issues?
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The other thing I can say about this, is if you are running 70 down the interstate and hit the throttle you get the lag and no power.of you stay on the throttle and drop back a gear the tachometer jumps up to 3k range like you would expect, and the lag is gone. If you dont drop back a gear and keep pushing the pedal further down eventually it kicks up a gear and it accelerates but it's much more dramatic than normal since u are really into the throttle. It cant be trans related though because the same effect happens when taking off from a stop sign. Right in that rpm range of low rpm it seems like that band is the issue. Like it has low power so the low rpm range struggles. To me this means fuel starvation or due to the ping I hear possibly a timing issue. I've never seen timing jump a tooth,but I have heard of it. Do you think this is a possibility? If it is I'm afraid its causing engine damage and I really dont want to wreck this engine. Still runs strong at 225000 miles
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post #6 of (permalink) Old 04-17-2019 Thread Starter
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One more thing I have been thinking about, sorry for all the info but I'm struggling to diag this ..... would the fuel regulator cause this issue and if so what's a good way to test it besides just replacing it? Thanks
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post #7 of (permalink) Old 04-17-2019
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^ Dunno? Does your '08 3.9 have an accessible/serviceable fuel filter?
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post #8 of (permalink) Old 04-17-2019 Thread Starter
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No. Does not have a fuel filter
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