Reduce engine power

Tiny
NAJERA962019
  • MEMBER
  • 2008 CHEVROLET EXPRESS
  • 4.3L
  • V6
  • 2WD
  • AUTOMATIC
  • 240,000 MILES
When on the highway and I slow down it will go into “reduce engine power” and won’t accelerate anymore I change the throttle body and pedal sensor, also spark plugs and cables and it keep doing the same. The code “p0451” came up took it to a mechanic and he changed the EVAP pressure sensor ran good for about two weeks and started doing it again.
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Wednesday, April 24th, 2019 AT 7:40 PM

8 Replies

Tiny
KASEKENNY1
  • EXPERT
Hi,

When you say EVAP pressure sensor, I assume you mean the fuel tank pressure sensor. If you look at the first chart in the attachments, you will see this DTC is set if there is high resistance in the 5 volt reference circuit, or the signal circuit. Also, it is set if there is a signal performance issue which would be the sensor. My guess is, this sensor was fine but there is a connection issue and by the tech removing the connector and putting back on the new sensor, he re-seated it and it worked okay for a while. Again, this is just a guess based on the info provided. I would suggest scanning codes again and confirm you have the same one and then testing the circuitry.

The other possibility is replacing the sensor did not fix it and the tech was able to clear the code. Then it took a little while to reset. The actual cause could be a restriction in the EVAP canister or vent lines could prevent fuel vapor pressure from bleeding off fast enough. If the vent system cannot bleed off pressure fast enough, the re-zero procedure may not complete successfully, which could cause this code to set.

Let me know what you find after you review these documents and we can go from there. Thanks
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Wednesday, April 24th, 2019 AT 8:48 PM
Tiny
NAJERA962019
  • MEMBER
I will try this in the morning. Regarding the codes since it goes into engine power reduce I have to disconnect the battery and connect again for it to accelerate again which caused it to reset I m guessing and now it does not throw any codes but keeps going into engine power reduce like four to five times a day. It s crazy when it happens in the highway, lol. Do you think the code has anything to do with this problem? Thanks
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Wednesday, April 24th, 2019 AT 8:58 PM
Tiny
KASEKENNY1
  • EXPERT
I think it is related. It may not be turning the check engine light on yet because it is what is called a multiple trip failure. This means it needs to see the same fault happen on subsequent key cycles before it turns the light on. However, the code might be there but when you disconnect the battery it clears the fault, then you drive it, it sees the fault and derates the engine. The code most likely goes pending (no check engine light) then when it sees it happen again, it will turn the light on and the code will go active. Either way, we need to monitor the system which driving to see what it is doing. Unfortunately, this is an issue that we need a scan tool in order to see what the module sees from the sensors.
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Thursday, April 25th, 2019 AT 10:00 AM
Tiny
NAJERA962019
  • MEMBER
Okay, I m located in Houston, Texas. Do you know or could recommend some that could help me with this issue? I have taken it to two guys but unfortunately they can t seem to find the issue or I m guessing they don t know. Both have charged be over $400.00 and it s crazy I can t seem to find the issue. Thanks
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Thursday, April 25th, 2019 AT 10:50 AM
Tiny
KASEKENNY1
  • EXPERT
Sorry for the delay. I was waiting on a response from a contact I have in the Houston region. I am in Virginia so I didn't know of any in that area. However, they recommended any of the Christian Brothers Automotive facilities in the Houston area. While I don't know these individual stores, I am familiar with this organization and in general they are very highly regarded.
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Friday, April 26th, 2019 AT 10:19 AM
Tiny
NAJERA962019
  • MEMBER
Awesome I will stop by. There s one like ten minutes from my house. I appreciate it a lot for your help. Just maybe do you have a rough estimate on how much you think this will cost? Thanks
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Friday, April 26th, 2019 AT 10:42 AM
Tiny
KASEKENNY1
  • EXPERT
Without knowing the exact issue, it is hard to say. It also depends on the facilities door rate but if it is a wiring issue as I suspect, it shouldn't be more than a couple hundred dollars. However, to diagnose it they should charge you a flat rate (normally one hour based on their door rate). So if their door rate is $100.00 per hour, they should charge you $100.00 and then tell you exactly what is wrong with a price to do that work. If they can't figure it out, then the proper business practice is to not charge you because they didn't diagnose it.

I am well aware that not all business' do this and they charge the customer no matter what but I personally never agreed with that. You are paying for a service which is for them to diagnosis it. If they do not diagnose it then they did not provide that service. They attempted to provide a service, but I wouldn't pay someone to come clean my house then they get here and say, "sorry I can't do it" and pay them anyway.
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Friday, April 26th, 2019 AT 10:54 AM
Tiny
NAJERA962019
  • MEMBER
Very true, thanks a lot for your help. I will check them out and hopefully get this issue fixed. Again thank you.
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Friday, April 26th, 2019 AT 3:21 PM

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