Hi everyone. I have a 2009 Silverado, 4.8l z71 4x4. For a while now I have been feeling the truck jerk during acceleration and you can feel a difference in power loss especially when trying to pass at highway speeds. It just feels like it doesn't want to get up and go. It will gear down and try to but just feels like it has no power. I know there is a large evap leak from the code being thrown and a tighten gas cap message on the dash. I have replaced fuel pump, fuel cap, charcoal canister, evap solenoid under the hood, and evap canister purge valve under the frame. Spark plugs look good. My question is, will a large evap leak cause this loss of power?

 

Just FYI - the purge valve is what’s mounted in the engine bay - the vent valve is what’s down by the canister.

The short answer is no, it shouldn’t. When you accelerate hard, the PCM shuts off the purge valve, so even if a large leak is present, it shouldn’t be affecting hard or WOT acceleration. If you wanted to be sure, you can unplug the purge valve for a few drive cycles and see if it makes a difference.

What you’re describing sounds like a restriction in the intake or exhaust. When’s the last time you checked the air filter?

If you have a vacuum gauge, check the reading at idle with the engine hot. 18-22 in-Hg with a steady needle is normal - lower than that or severe oscillation of the needle indicates base mechanical problems.

 

Thanks for the reply and info.I keep forgetting the actual names of the evap valves lol. It hasn't been very long since I've changed the air filter. I do not have a vacuum gauge but maybe I can get my hands on one. Could it possibly be a fuel injector, ignition coil or maybe a clogging catalytic converter? It idles fine though and I figure it would throw an o2 code for catalytic converter

 

Another potential helpful piece of information. About a week ago I noticed the check engine light had went off and it felt like it was performing normally. On the same drive the check engine light came back on and it had the loss of power again. I didnt get the check engine light checked that day but the last I have checked the only thing showing was an evap leak.

 

A loss of power isn’t generally noticeable with a single cylinder misfire (faulty injector or coil) - just a hesitation or “jump” when the miss occurs. Multiple cylinder misfires could certainly result in a lack of power, but typically the check engine light will start flashing because the misfires are damaging the catalytic converters.

Speaking of which, that’s why I suggested checking vacuum, because a converter that’s breaking down can cause a restriction in the exhaust. Restrictions to airflow (intake or exhaust) can cause low vacuum in the intake, so checking it can often be a quick and easy way to determine if the exhaust should be checked more thoroughly.

Oftentimes idle and light load conditions seem normal with a failing converter, but when heavy acceleration is needed, the restriction prevents full airflow and the engine can’t provide the necessary power.

That said, if it still only has the large EVAP leak code, it’s possible the tubing connecting the canister and purge valve is cracked or broken and the purge valve opening is drawing unmetered air into the engine instead of fuel vapor. I would expect other codes to be present (MAP performance or system lean codes for example) if that were the case but it wouldn’t be the first time a PCM had me scratching my head.

 

I gotcha. I can rent or find a vacuum gauge when I get the chance. I'm really starting to think it is the converters though because I do smell a strong sulfur smell. I also checked the temperature with a sensor today and the results before and after the converter were roughly the same. I believe the temp should be a good bit warmer after the converter right? I will also look into the tubing for the evap leak. I also did unplug the purge valve under the hood and it didn't help any

 

My 2014 GMC is a diesel. That's why I'm on this forum. But I also have a 1990 GMC 5.7 pickup that was doing what your is...no power under acceleration. It turned out to be a clogged fuel filter. Just a thought.

 

I replaced the fuel pump not long ago and it has the filter built in with it but I appreciate the tip

 

I bought a vacuum gauge and it failed the test so I'm going to replace the catalytic converters. I'm planning on buying the complete setup so I can just bolt it right up. I can't afford OEM so is there a certain brand you suggest that won't cause any issues? Im not wanting to go too cheap either. I was thinking magnaflow or flowmaster but not very familiar with them

 

Given the lack of power and sulfur smell you describe, as well as the vacuum test results, the cats are the most likely culprits. If you want a little more certainty, one other test would be to remove the upstream (closest to engine) oxygen sensors and go for a drive. Obviously you’ll have some extra noise, but if the cats are breaking down, you’d be giving the exhaust some relief from the back pressure, and power should improve.

As far as brands, Walker is the only one I’ve had any experience with. At work we don’t do extensive exhaust repair and refer customers to an exhaust shop we work with. Any time they replace converters they install Walker and swear by them.

We will also occasionally buy exhaust clamps and other necessary hardware/adapters/extensions as needed. Most of the stock kept by our primary supplier is also Walker and my opinion is they make good products.

Considering that converters don’t break down unless there’s excessive exposure to unburned fuel, oil or coolant, it really shouldn’t matter what brand you choose because they’re all built to the same specifications depending on your vehicle. That’s not to say there’s no questionably manufactured converters out there, but generally I wouldn’t stress about it too much - go with the name you trust.

That said, I don’t have any direct experience with magnaflow, flowmaster, or any other big aftermarket names. The work I do is pretty straightforward - part comes off, same (replacement) part goes back on, though not always in the right order. The mod and performance seekers in this area typically enjoy doing that work themselves.

One last thing to note - While I can appreciate needing to keep a budget, I’d really recommend professional installation unless you’re a very skilled welder. Besides most emissions states requiring converters to be welded in, the best way to keep the cats working efficiently (and to keep the CEL off) is to make sure they’re completely sealed. Welding is the best chance of accomplishing that.