Vapor lock issues?

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?

Image (Click to make bigger)

5 Replies

JACOBANDNICKOLAS

MECHANIC
108,128 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

SPONSORED LINKS

Was this

answer

helpful?

Yes

No

Monday, May 22nd, 2023 AT 8:31 PM

TWEBB101

MEMBER
157 POSTS

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.

Was this

answer

helpful?

Yes

No

Tuesday, May 23rd, 2023 AT 10:04 AM

JACOBANDNICKOLAS

MECHANIC
108,128 POSTS

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.

Images (Click to make bigger)

Was this

answer

helpful?

Yes

No

Tuesday, May 23rd, 2023 AT 8:44 PM

TWEBB101

MEMBER
157 POSTS

Sorry Joe, no luck. Attached are what I see underneath.

Images (Click to make bigger)

Was this

answer

helpful?

Yes

No

Wednesday, May 24th, 2023 AT 9:06 AM

JACOBANDNICKOLAS

MECHANIC
108,128 POSTS

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