No codes, power loss

[blackhawk17]

Hello all.

I am attempting to troubleshoot my wife's 05 Tahoe from Afghanistan while the mechanic can't figure it out.

So it has a loss of power at highway speed, uneven acceleration, an occasional miss, and sometimes balks when taking off from a start.

The problem does not always present, and it's not throwing any codes. The mechanic says he has ruled out just about everything other then maybe a knock sensor or something internally. He's waiting to see if it's the knock sensor, and if not wants to tear into the engine if that's what I want.

Has about 170,000 miles and for the last 15,000 had been doing a lot of heavy towing. Always well maintained. Rebuilt transmission 15,000 miles ago. Problem has just started in the last month.

I guess my question is if the knock sensor could be faulty and still not throw a code, and retard the timing and cause that power loss and the miss. He thinks so even though there's no code.

Sorry for the lengthy post. Thanks for any advice.

Matt



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[blackhawk17]

From what I have read, some of these vehicles have in line fuel filters and some have a co-located pump and filter assembly. Anyone have experience changing either?

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[Chip18]

Hey ... I get to be the first to say ... "Thank You For Your Service!"

That said ... it's still early and hopefully more knowledgeable members will be chiming in later! The "knock Sensors" would pull power timing and thus cause a loos of power but I'm almost certain they would throw a code if that were happening??

I'm new to these vehicles myself but I don't think there is a separate "Fuel Filter" as such??? I think those went away in 2000 on most/all FI engines?? Everybody seems to put the little sock thing on the in tank fuel pumps and calls it good??? I'd "assume" the mechanic checked the fuel pressure??? A bad/failing fuel pump could do this and I don't think that would throw codes??

The Knock sensor "not throwing codes" is kinda weird if they are failing ... but we just had a member that used the fancy Snap On stuff and he was able to "see" that the motor was pulling timing! And it was indeed a bad knock sensor!

See here:

http://www.tahoeyukonforum.com/threads/huge-hesitation-mechanically-inclined-please-help.84229/

 

[Rocket Man]

From what I have read, some of these vehicles have in line fuel filters and some have a co-located pump and filter assembly. Anyone have experience changing either?

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Yours has a filter in the tank with the fuel pump. You have to drop the tank to get to it. There's threads here with more info, you'll have to search.

 

[iamdub]

If the PCM doesn't see a problem with what it's being told, it's not gonna throw a light. A faulty knock sensor can erroneously detect knock and report this to the PCM and the PCM is just gonna respond based on how it's programmed, which is to retard the timing. As far as it's concerned, it's just doing what it's told to do when it gets certain info from a sensor. That info is still within the allowed parameters so it's gonna assume everything is operating as it should so there's no reason to throw a code/SES light. It doesn't know that the knock sensor is faulty and lying about the engine knocking. This same scenario is true for any other sensor on the vehicle. The problem may not even be retarded timing. An SES light is helpful because it can at least point you in the direction of the system that the PCM believes is faulty. With an SES light/DTC, the PCM stores a freeze-frame that will show what all of the sensors were telling the PCM when the PCM set the code. This info, if understood correctly, can help to even further pinpoint the root of the problem. Maybe the Throttle Position Sensor is telling the PCM that the throttle is opened to only 20 degrees of angle when your wife actually has the pedal floored, so the PCM is adding the appropriate fuel needed for when the throttle is opened on 20 degrees. Or maybe she has the pedal floored but the TAC module is telling the PCM to open the throttle to only a fraction of that. Again- the PCM doesn't know the TAC module is lying. Or maybe there's a stuck and leaking fuel injector(s) causing a rich condition in a cylinder(s). See where I'm going with all of this? A competent mechanic would read the real-time live data stream and see what info the sensors are telling the PCM and what the modules do when the PCM responds to that info from the sensors.

Case in point: First off- I'm not a mechanic and I don't have a $2,000 scan tool. I have a $250 scan tool I purchased off eBay. My neighbor mentioned he was getting quotes on a transmission rebuild for his 230,000+ mile Silverado because it was randomly bogging and shifting strangely, but only at certain throttle amounts. He had no SES light/no codes, religiously and meticulously maintains his truck, and in no way abuses it. I hooked up my scan tool and read the live data stream while he drove. I noticed that, at a cetrain range of throttle input, the MAF had a spot where it reported a sudden drop in airflow into the engine but then it jumped back up. We went home and removed the MAF. The sensing elements were totally black from being covered in contaminants. He said he never knew anything about periodically cleaning the MAF sensor or he would've done it. I cleaned the MAF sensor while he cleaned the throttle body. He said his truck feels peppier and the trans hasn't had any odd shifting since.

 

[jfoj]

Has about 170,000 miles and for the last 15,000 had been doing a lot of heavy towing. Always well maintained. Rebuilt transmission 15,000 miles ago. Problem has just started in the last month.

Here is my take, with this kind of mileage, I would replace the fuel pump and filter out of a Preventive Maintenance measure anyway. Unfortunately you are not in town, but not sure you would do the work anyway.

Fuel pumps do not last forever and when they hard fail, you are left stranded. Not sure the Tahoe/Yukon/Suburban had problems with the fuel pump wiring at the sending unit like some other earlier models, but this is also another possibly problem area that can only be checked once the fuel pump and sender is removed.

I have seen fuel pumps on many manufacturers "soft fail" due to worn brushes and armatures where this causes reduced fuel pressure and volume. The engine will run, but not like it should. Replacing the fuel pump and filter is somewhat necessary at this age and mileage anyway and it will remove these as a possible source of the problem. Far better than tearing into the engine.

A word about scan tools, OBDII Apps for smart phones are DIRT cheap these days. I would recommend you have your wife buy one and the interface. Not sure if she would be willing to work with the tool, but they are pretty easy to set up and they are AWESOME for gathering data that can be emailed and uploaded to the Internet for remote viewing. If she is not capable or interested, maybe you have a buddy back home that can help.

You can set up Logging on these tools and then see every PID and what values are available while driving. Everything from vehicle Voltage, O2 sensors, Fuel Trims, Ignition Timing, Coolant Temperature and Throttle Angle. This call can be graphed as needed.

I HIGHLY suggest the OBDFusion App and a VeePeak interface from Amazon. We are talking less that $30 for both, if the platform is Android, it is less that $20. Do not let anyone put these tools down. I have a number of Professional level scan tools from Autel and SnapOn and these Apps are my first go to tool and they can do things with gathering and sharing data that the Pro level tools cannot do.

Android requires a Bluetooth interface, iProducts requires a Wifi interface.

I can provide more info if interested.

 

[blackhawk17]

Yeah, I do intend to get a scan tool. But, it is confirmed to be the fuel pump, which was my original suspicion before she ever took it to the shop. I wish I was there to do it, I hate paying for labor...

Thanks for the input guys.

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[blackhawk17]

That app sounds cool. I think I'll do that. Thanks!

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[blackhawk17]

Ok, so fuel pump replaced- problem not fixed. He concluded that the pressure was good, but the volume was not.

Sooo, same problem, $900 later. Yay.

Maybe I'll have the MAP sensor looked at. I did install a CAI earlier this year with an oiled filter. Who knows, maybe that's it.

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[jfoj]

Sorry the fuel pump did not resolve the problem. I hope the fuel pump replacement was not $900. If it is, I would like to understand the cost breakdown. Seems high to me, but I also do not know the city and state were this work is being done.

So if the systems are still the same, assuming there is no proper problem like a flaky fuel pump relay with too much Voltage drop, then I would think a "DECENT" mechanic could use ever a very basic OBDII tool to monitor Live/Realtime data to watch Fuel Trims and Post Cat O2 senor behavior to look for things like Lean conditions, clogged catalytic converter(s), throttle problems, MAF reading that may be dropping out etc. A generic OBDII tool like OBDFusion would allow this. BUT what I see all the time is "mechanics" buy the $6k+ Snap-On Scan tool, and totally skip the OBDII section and jump right to the "Scan" portion of the tool which is usually manufacturer specific and enhanced data. BIG, BIG MISTAKE, but these guys should be behind a counter selling parts of this do this IMHO.

Fuel Pump relay should be cheap and easy to replaced and again rule out any obvious fuel problem.

I hate throwing parts at cars, but this is what may happen with this current mechanic.

I use my $4 OBDII App ALL DAY LONG, set it up to Log, go drive and then I can come back and dump the .CSV file. I open the file first in table view and quickly scan for obvious problems visually, then I typically will start to graph Fuel Trims, O2 sensor Voltages, Engine Coolant Temps, Fuel Trims, Charging Voltage and so forth until I start to see a possible problem.

The one sensor than can really give headaches that often will not trigger codes but impact driveability can be a flaky crank sensor. I do not think these engines have known crank sensor issues, but these can often cause flame out problem, misfires and no starts.

Not sure what to tell you, you may need to speak with the "mechanic" and find out if he has the mind to look at the bigger picture or just expects codes to tell him what part(s) to replace.

I understand how frustrating this type of problem is when you are not able to get directly involved and are located half way around the globe.