1995 Blazer - 4.3 L Vortec V6 CMFI (vin W w/ PCM)

230,000 miles

There are no trouble codes. This is a OBD-I or 1.5 system from a GM transitional year (I have the OBD-II style connector).

I have issues that are possibly related to the Evaporator Canister Purge system...

  1. Occasionally, I get "dieseling" (run-on) after shutting off the truck. See edit 1 below.
  2. I occasionally smell a slight amount of gasoline fumes when driving with the windows down.
  3. After fixing a lot of other things1, I still occasionally have some slight lack-of-power issues.

The gas smell seems to be coming from the rear of the truck. When I remove the tank vent from the canister (the middle hose; from canister to tank), a little gas drips out. I know that's not right. Visually, I see no leaks from any of the lines or the canister.

The GM service manual also has a page for "dieseling" in the drivability section. It points to the Evaporator Purge system. Edit: This has been ruled out.

So what causes gasoline to get into the vent line? I'd rather not change the canister without knowing why fuel is getting into it. Does gasoline in the canister explain the engine run-on? I've read about ways to bypass or eliminate the canister, but these techniques also cause a gas smell. Are there any definitive tests that will narrow down the source of trouble?

FWIW, this vehicle is not subject to emissions testing.

1 "after fixing a lot of other things" refers to a rough idle condition that led to checking everything... spark plugs, wires, compression testing, timing chain slack, etc. Ultimately, the rough idle was fixed by setting the timing back to factory spec (it was 10 degrees retarded)... still not right... so then changing the entire distributor (bad bearings) and setting timing to factory spec ultimately took care of 90-95% of the rough idle. (Edit: Root cause of rough idle ended up being a pinhole leak in the gas return line inside the intake plenum.)

Fuel pressure leak-down test was performed many times about a month ago while chasing down the rough idle condition. Pressure held steady for more than 20 minutes at 58 psi. This would rule out any/all leaks from pump to CMFI "spider". (Edit: Wrong!! It only rules out the gas feed side and the "spider". However, if you have a leak in the fuel return side coming out of the "spider" inside the plenum, a leak-down test will be completely useless. This was exactly my problem!)


I removed the top hose (from canister to engine) and drove around a bit. I still have the run-on, which presumably rules out the canister as the cause.

EDIT 2: This was flagged as a possible duplicate. However, these two questions are not the same. This question specifically is asking about how the Evap Purge system operates, a gasoline odor, why there is gasoline inside the canister, symptoms of failure, and what can/should be done about it.

The other newer question specifically delves into the possible causes of a dieseling condition, what's been recently done, and what else can be done.

  • You should get a fuel pressure reading on this. It appears the spider is a common issue, where fuel pressure will kind of be all over the place, depending on what you are doing with the vehicle. It will also tell you if there is a problem with bleed off, which can cause other issues. Jul 22, 2015 at 21:39
  • What does "After fixing a lot of other things" mean exactly Jul 22, 2015 at 21:42
  • @Paulster2, sorry I did not get into the history too deeply as it would take ten pages. The spider was changed a few years back and the fuel pressure readings tested good just a couple weeks ago. No issues with fuel pump or spider. Reading 58 lbs. and practically zero pressure drop over 20-30 minutes. This question is really about the evaporator purge canister... why fuel is getting into it and what I need to do to properly test and fix.
    – Sparky
    Jul 22, 2015 at 21:45
  • @Movemorecommentslinktotop, it means that practically everything from timing chain slack to knock sensors have been fixed or ruled out. So many things have been replaced and inspected that it would be easier to address them as they come up than list them all in the OP. However, I have very specific questions about the evap purge system.
    – Sparky
    Jul 22, 2015 at 21:47
  • Which hose has fuel in it? Jul 22, 2015 at 21:51

3 Answers 3


I removed the top hose (engine to canister) and drove. I still experienced the run-on, so I believe this conclusively rules out the canister as the cause of the dieseling.

I also found that the connection of this top hose to the metal tubing was very loose and possibly the cause of the gasoline smell. I put a hose clamp here to eliminate this.

As far as why there is gasoline going into the canister through the tank hose, this part is still unknown.


There is a check valve in the vent line where the line sits in the gas tank. This valve, Looks like a ball bearing and a seat, closes the line off unless there is pressure. This allows the vapor, and vapor only, to travel to the canister. Some liquid will condense, but it should never be full of gas.

  • So why is it full of gas?
    – Sparky
    Jan 30, 2017 at 14:44

I am having this exact issue with gas being in the lines and I previously had the canister close valve and charcoal canister replaced. The mechanics are saying that it is my fault gas is getting in the lines because I must have tried to overfill (top off) the gas tank. I do not do this but if you do it may be your cause. The other thing is that I have read the valve itself can be faulty and cause this so I am pretty upset that they would just come out and claim that I top off and it could not be a fault with their part. So I suppose for you as well it could be the part is faulty.

  • We will never know. I never fixed it and junked the car at around 250,000 miles. Looking back, I think it was the CMFI spider, but I had the opportunity to get a used Silverado and didn't want to put another dime into that Blazer.
    – Sparky
    Jun 25, 2023 at 4:46
  • How is the Silverado doing? I hear all sorts of up and down information about Chevy in general but may be needing a newer car soon :(.
    – Helloworld
    Jun 25, 2023 at 14:47
  • It's great. It's a 2000 and no complaints other than the rusted out rockers.
    – Sparky
    Jun 26, 2023 at 1:04
  • That's awesome, I've heard those years where still pretty good.
    – Helloworld
    Jun 26, 2023 at 14:50

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