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Vapor lock issues?

Tiny
TWEBB101
  • MEMBER
  • 1984 CHEVROLET TRUCK
  • 7.4L
  • V8
  • 2WD
  • AUTOMATIC
  • 170,405 MILES
I think I'm having some vapor lock issues, and was wondering if you could help me with a fix? Truck runs great, but lately, as the temperatures have warmed up, if I sit in traffic, or idle for a few minutes, or stop and try to start the engine again, it will just shut off, sputter/choke like it's starved for fuel, or won't start unless I hold the pedal down for a bit. I've attached a diagram of the current fuel system setup. I'm planning on getting a heat sleeve for the fuel line, but anything else I need/should do?
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Monday, May 22nd, 2023 AT 2:38 PM

21 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,758 POSTS
Hi,

It could be a vapor lock issue, but there are other things as well that are affected by heat. If you haven't already, I need you to check for diagnostic trouble codes. This truck has an OBD1 system that doesn't require a scan tool. All you need is a short jumper wire or even a paper clip.

Follow the directions in this link:

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

First, I assume this is a C-20. Is that correct? Also, have you had any issues in the past when it was warm outside? Is the fuel line set up the same as it was from the factory?

If that is the case, the heat sleeve is a great idea anywhere the fuel line is near excessive heat such as an exhaust manifold. However, I want you to try something. When it doesn't start, see if it will if you use starting fluid. If it does and then stalls, then we know it is a fuel-related issue. If there is no change, then it is likely ignition related and not a vapor lock issue.

Let me know.

Joe
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Monday, May 22nd, 2023 AT 8:31 PM
Tiny
TWEBB101
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Hi Joe! I can't seem to locate that OBD1 switch; I don't know if it's been removed, or if it's somewhere else in the truck, but there isn't anything under the dash for it.

Yes, it is a C20, and yes, the fuel system should be factory set up again after my rebuild. Only thing missing is the second fuel tank/selector switch, which I've bypassed, going directly from the sending unit to the fuel pump, with an inline filter in-between.

I don't remember having any issues last year during the warm months, but maybe it did and I just didn't pay attention or worry about it. That's not helpful, I know.
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Tuesday, May 23rd, 2023 AT 10:04 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

It should be below the steering column. I attached a pic below showing the location or at least where it should be.

Let me know if it's there.

Take care,

Joe

See pics below.
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Tuesday, May 23rd, 2023 AT 8:44 PM
Tiny
TWEBB101
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Sorry Joe, no luck. Attached are what I see underneath.
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Wednesday, May 24th, 2023 AT 9:06 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

On some of these, they placed it on the passenger side. Did you check there as well? Also, if you are still experiencing the issue, do me a favor. When it stalls and doesn't want to restart, see if it starts if you use starting fluid. If it does and stalls, the problem is related to the fuel system. If there is no change, check for spark at the plugs.

Here is a link that explains how it's done:

https://www.2carpros.com/articles/how-to-check-for-ignition-spark

Let me know what you find.

Joe
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Thursday, May 25th, 2023 AT 6:56 PM
Tiny
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Hi, can I reopen this thread? For reminder, I have a Quadrajet carbed 454, basic factory setup: stock mechanical fuel pump with heat shield-wrapped rubber fuel line (away from the block), factory-style tank and factory style sending unit with 2 fuel lines running the frame rail of the truck.

Two issues/questions:

- When the outside temperature is hot, after driving the truck for 20 mins or more and stopping, I can hear hissing coming from the tank/sending unit. The sending unit is the factory-style, 3 outlet kind, with the middle outlet as the vent. Is the hissing sound normal for hot days, or is something not working right?
- Also on hot days, after driving the truck for awhile, sometimes the engine will just die. Generally happens after driving, and then sitting in a drive-thru, sitting at a stop light, etc, and then when I try to accelerate again, it dies. I can immediately start it again by cranking the engine and pressing the accelerator pedal about half-way; when it starts, I rev it a couple of times, and I’m good. Is this vapor lock, or something else?

Any tips on either of these?
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Sunday, June 9th, 2024 AT 2:07 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Usually, a vapor lock won't cure itself that quickly. It requires time to cool.

Is this vehicle the same year, make, model, and engine size as the original post? Let me know if it is. If it isn't, I will need you to start a new post. We try to keep them specific, so they are more helpful to others. I hope you understand.

If that is the case, just copy and paste your question to a new post.

Here is the link to where you can do it:

https://www.2carpros.com/questions/new

Let me know.

Joe
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Sunday, June 9th, 2024 AT 8:51 PM
Tiny
TWEBB101
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Thanks, yes this is the same truck: 84 C20, 454, quadrajet carb. Same details as original post.
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Sunday, June 9th, 2024 AT 9:20 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

Have you checked to see if there are any codes stored? If not, here is a link that shows how to retrieve them:

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Also, if you have the same issues, does it start if you hold the pedal to the floor? If so, do you ever notice if there is dark smoke from the exhaust when it starts?

Let me know.

Joe
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Monday, June 10th, 2024 AT 7:46 PM
Tiny
TWEBB101
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  • 178 POSTS
I don't have an OBD1 port anywhere in the truck, so I can't check for codes. Yes, it does start when depressing the accelerator pedal, and I don't notice anything abnormal from the exhaust.
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Monday, June 10th, 2024 AT 7:56 PM
Tiny
JACOBANDNICKOLAS
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Hi,

If it starts with the pedal to the floor, it sounds like it is flooding. Doing this opens the throttle plate fully and allows more air to enter, clearing a flooded engine.

Pay attention to see if you see anything as far as smoke and if the smell from the exhaust is strong, which may indicate a rich fuel mixture.

Let me know.

Joe
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Monday, June 10th, 2024 AT 8:17 PM
Tiny
TWEBB101
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Okay, I'll try to watch for that when it happens again. What would cause it to flood after driving for a while and then idling for a few minutes? Also, any thoughts on the hissing from the fuel sending unit?
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Monday, June 10th, 2024 AT 8:22 PM
Tiny
KEN L
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It sounds like there is a problem with the fuel lines on the suction side, even though they don't leak they can allow air to enter the fuel system which creates the problem. I would replace all rubber lines back at the fuel tank and the suction side of the fuel pump.
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Monday, June 10th, 2024 AT 8:22 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

The fuel sending unit in the tank? The truck has a mechanical pump, so if you hear a hissing, chances are there is a leak, and you hear the vacuum from the pump leaking.

Actually, I believe Ken just nailed it. Thanks, Ken.

Let me know.

Joe
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Monday, June 10th, 2024 AT 8:24 PM
Tiny
TWEBB101
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Thanks to both of you. It has rubber lines from the tank to the steel lines, then from the steel lines to the pump, and rubber line from pump to carb. I’ll check them for holes or look at replacing them.

As for the hissing, it’s definitely coming from the sending unit, the middle of the three inlet/outlet tubes on top of the sending unit.
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Monday, June 10th, 2024 AT 8:30 PM
Tiny
KEN L
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I would remove the tank to see if there is an obstruction at the pick-up filter (strainer).
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Tuesday, June 11th, 2024 AT 10:24 AM
Tiny
TWEBB101
  • MEMBER
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I replaced all the rubber fuel lines with new rubber, and new hose clamps, the fuel injection kind (got rid of the worm drive clamps). Took it for a drive today (about 95 degrees outside temperature) driving, it works great. Stopped for about 30 minutes, it started right up, but slowing moving through a parking lot, it shut off again. Gave it some throttle and it fired right up and ran great. Same story later on, but after sitting for a couple of hours, it shut off again on idle after starting up.
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Tuesday, July 9th, 2024 AT 7:53 PM
Tiny
TWEBB101
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@Ken L or @JACOBANDNICKOLAS, do you have any other ideas?
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Monday, July 15th, 2024 AT 7:15 AM
Tiny
KEN L
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Yes, a bad fuel pump. I didn't read where you replaced it? Dirt and debris can get in the flap valve and cause low pressure which leads to the vapor lock.
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Monday, July 15th, 2024 AT 10:05 AM
Tiny
TWEBB101
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Thanks: yes, I have not replaced the fuel pump since it was put in new a couple of years ago. I'll look into replacing that. Could a carb spacer help with the heat soak?
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Monday, July 15th, 2024 AT 10:34 AM

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