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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!)


EDIT 1:

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. – Pᴀᴜʟsᴛᴇʀ2 Jul 22 '15 at 21:39
  • What does "After fixing a lot of other things" mean exactly – Move More Comments Link To Top Jul 22 '15 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 '15 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 '15 at 21:47
  • Which hose has fuel in it? – Move More Comments Link To Top Jul 22 '15 at 21:51
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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.

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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 '17 at 14:44

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