I Experienced Vapor Lock on a TBI

[L31MaxExpress]

I started with the 4.3L open loop AFR adder calibration and ended up adding even more fuel in open loop than it did. I forget if that was from a GMT800, S10, fullsize van or M-van calibration but seeing the adder used made me play with it as well.. With the IATs in the 180-220F range on a hot restart my open loop commanded AFR is in the low 10s to get near a stoich AFR on a hot re-start and even then it is still sometimes a bit lean.

 

[L31MaxExpress]

I will also add that the problem is much less noticeable on the Proflow 4 fuel rail setup where fuel flows from the end of one rail, through a crossover on the end to the other rail and exits the opposite end of the rail. The prime pulse pushes cooler fuel through both rails on that setup and purges the hot fuel back to the tank within a few seconds on startup.

The TPI setup I had in my old 1983 G20 had the same kind of issue with fuel stagnated in the rails on a hot restart. At the time I did not understand why it would end up running so lean on a hot restart, surge and carry on for a short period of time.

The Holley fuel rail that is about to end up on my 97 has a similar setup to the Edelbrock so I will get to see how it works next summer. Doubt we will have weather hot enough to see the issue until then. The Holley rail has a crossover on both ends, but fuel enters on one side and exits the other thus hot fuel will not be trapped in the rails like it is on the current Mercruiser fuel rail.

 

[Coveman]

High altitude can be a contributing factor to vapor lock as well. I remember going over a pass near Telluride one summer and some folks from Texas had rented a gmt400 burb to go over the Imogene. They were kinda freaking out when I got to them (at the summit) and I explained that they just needed to let it cool down a bit and it’d be fine (used to happen to our school buses when I was little) Sure enough 20 minutes later it fired up and they were on their way.

 

[Pinger]

* Vortec -- Higher pressure then TBI -- and also pressure is now held between trips.

Furthermore, in addition to the
2-second prime with the initial Key ON, the Vortec systems have an additional 2-second
fuel pump operation at Key OFF. (!) My unproven theory is that the designers were
trying to ensure that a hot fuel module wouldn't be necessary on the 8-lugger Vortec
GMT400s.

That explains something that hitherto confused me. I have an AFR gauge and its power is tapped from the signal from the ECU to the fuel pump relay. For a cold start I often cycle the key a couple of times (as when it last ran it was on LPG/propane) and on shutting the key off I see the AFR gauge light up again. Now I know why.

No mention of accumulators - which are (or were) commonplace on EFI cars to inhibit vaporisation in the fuel lines. None on GMT400s - or have I just not noticed mine?

 

[Scooterwrench]

Easiest fix is to hold the throttle wide open and bump the starter several times. Every time you bump the starter the fuel pump will prime for 2-5 sec. and clear the vapor. If you don't hold the throttle WO it may prime the intake and create a flooded state. I just looked at the .bin file for a 1227747 ECM and didn't find a fuel pump run time before crank. The PCM I've put in my 91 has that parameter with a factory set time of 10sec. which I can increase or decrease at will.

 

[BC99]

Just a thought. My truck is a 1999 K2500 small block. Not long after I got it I experienced severe vapor lock symptoms during the first summer. It didn't repeat but reoccurred the next summer as soon as it got hot out. It seemed similar to an issue going back many years on a family vacation. My Dad's '76 Ford Granada crapped out on the drive through the mountains of BC on a hot day. It turned out to be a gas cap issue. Changed the cap and drove on to Ontario issue free. I know the fuel cap is a much more important part of the fuel system on today's cars. I thought, what the heck, I'll try it on the truck. Cheap fix. Never had an issue again. A couple of years ago I was working with a millwright and he was having again, a similar issue with his '2000 Tahoe. He laughed at me when I suggested that he replace his gas cap. I joked that in my opinion GM made lousy gas caps. I convinced him that it was a cheap repair that could do no harm. Worst that could happen is you get a new cap. My friend didn't like taking advice from the service and parts guy but he asked me to order him a cap on the next parts order that day. Yup, no more issues. He felt good again sending his wife and kids out in the Tahoe once more. His wife felt good that she didn't have to drive his rusty Jetta anymore as well.

 

[Road Trip]

Just a thought. My truck is a 1999 K2500 small block. Not long after I got it I experienced severe vapor lock symptoms during the first summer. It didn't repeat but reoccurred the next summer as soon as it got hot out. It seemed similar to an issue going back many years on a family vacation. My Dad's '76 Ford Granada crapped out on the drive through the mountains of BC on a hot day. It turned out to be a gas cap issue. Changed the cap and drove on to Ontario issue free. I know the fuel cap is a much more important part of the fuel system on today's cars. I thought, what the heck, I'll try it on the truck. Cheap fix. Never had an issue again. A couple of years ago I was working with a millwright and he was having again, a similar issue with his '2000 Tahoe. He laughed at me when I suggested that he replace his gas cap. I joked that in my opinion GM made lousy gas caps. I convinced him that it was a cheap repair that could do no harm. Worst that could happen is you get a new cap. My friend didn't like taking advice from the service and parts guy but he asked me to order him a cap on the next parts order that day. Yup, no more issues. He felt good again sending his wife and kids out in the Tahoe once more. His wife felt good that she didn't have to drive his rusty Jetta anymore as well.

Exactly. Pre-emissions we used simple bi-directional vented caps so that a vacuum wouldn't be
developed in the gas tank while the fuel pump was busy emptying it -or- have excess pressure
develop as a full tank on a parked vehicle warmed up over the course of a hot day.

But in order to help control the evaporative emissions, our gas caps were upgraded by installing
small one-way air valves. I think of them as an 'air diode', in that air is allowed in as needed, but never
allowed back out. (Relieving excess pressure is where the charcoal cannister gets involved.)

The point is, just like diodes in the land of electricity, these one way check valves in our gas
caps can & do fail, and when they do, they can definitely cause 'vapor lock' (ie: no fuel delivery)
symptoms.

Good on you for sharing your experiences. Staying on top of these intermittent failures is
how we can keep our aging rides in the DD or better level of reliability.

Cheers --

 

[wb292]

I’ve experienced vapor lock and stumbling at red lights many times on all four of my trucks. But I am an extreme case. I drive 68 miles each way to work through Mohave Valley AZ. It’s not uncommon to reach the mid 120’s every summer. I solved it on my 1991rcsb with a few cooling mods. First swap to a 180deg thermostat. Preferably a balanced sleeve type. Like a 330-180. I then installed older style intake manifold gaskets with the heat riser block off plates. This helps keep EGR heat away from the intake. Don’t forget to use the coolant passage block off plates at the back of the intake also. Then swap the 5 blade fan for a 11 blade from a 96 and newer 350 and a severe duty clutch. Then replace the water pump with a high volume high pressure one. Next replace the radiator with the taller 34X19x2 inch core size. You will also need the taller fan shroud. Your AC will preform better with the extra air flow too. Now this truck was not overheating at all before I did this. Just vapor locking in the afternoons. With these lower under hood temps the vapor locking and stumbling stopped.