Rich Fuel Both Banks at all times

Michael Mobile Technician Utah Posted   Latest  
Case Study
2001 Chevrolet Silverado 1500 5.3L (T LM7) 4-spd (4L60-E)
P0300 — Random/Multiple Cylinder Misfire Detected
Poor Fuel Economy

I was called to a shop to help them investigate a vehicle they had just done cylinder heads on. It was running -25% Combined LT and ST fuel trim. It was setting codes for rich mixture and random misfire. When looking at scan data, it was not making sense. We did find a spark plug that was not firing well. The problem moved with the plug. We checked all the usual suspects. EVAP system, Fuel Pressure Regulator, MAP and MAF. The vehicle was just getting too much fuel all of the time. I reflashed the PCM thinking maybe it had the calibration for a different engine. The result was a slight improvement. Still no joy. I decided to go on with my day and do some research later in the evening. A few hours later, the shop owner called and told me that the customer had replaced his injectors with some off the internet. I guess we have our answer. Good old DIY ingenuity. The vehicle had some lean codes. The customer's solution was to put in larger injectors to make up for the intake leaks. Once the leaks were fixed with the valve job, now it goes rich. Having the vehicle history makes all the difference.

Allan Instructor

DIY logic, it's cheaper to buy and replace a set of injectors to add more fuel to a lean running engine than it is to repair the vacuum leak that caused the lean condition.

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Cuba Mobile Technician

Have to adore the customers that fail to give you the full story, whether because they forgot or due to avoid looking inferior because they threw parts at something and it didn't work. There's also those who damage more when DIYing. I have no issues with people wanting to do their own work or learning, just try to be forthright when unsure of things or of something they did. Great find. You

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Jim Mobile Technician

I just talked in GREAT detail about situations like this. This is BY FAR the main reason that I developed my interview sheet. If we don't ask questions, it can cost TIME and MONEY. Also, if we do ask questions, like any other repairs recently and don't get the correct answer, when we find something like this , it will justify added diagnostic charges.

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Luis Technician

Hi Jim would you be willing to share your interview sheet?

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Geoff Diagnostician

Vehicle history would sure be nice. We never get it here. Everybody always "just bought it". Who buys a 15 year old POS with a driveability issue?? Everybody here of course! I've seen wrong injectors on Ford a couple times when "they" swap in the regular-fuel used engine in place of the FlexFuel one. Never knew who they was, but a few guys have ALWAYS been at these things by the time it gets

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Sean Owner

Last year, I had an 2007 Expedition 5.4l that was running very rich. Fuel pressure dead on at 60, everything tested good. Finally, on a whim, I looked at fuel pressure specs for same year F150, and it called for 35 psi. I looked up the injector pictures in an electronic catalog, and found the F150 injectors have a brown band, and the Expedition injectors have a pink band. Installed new

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G Educator
New York

That's why we all need to investigate the vehicle and ask the owner where and what has been done. Nice information....thanks for sharing

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