InfernalVortex

iTrader: (4)

Im not quite sure what to make of this. I have some theories, but I was wondering if you guys had any experience with anything like this.

The car is just a vortec 355 with a Holley double pumper and a TBI pump regulated to 6 psi with a mallory return regulator.



The problem:
At our local club, we get 8 runs. 4 morning runs, 4 afternoon runs. First two runs were great. 3rd run I had some weird stuttering near the start. 4th run, it was a little worse and the car was running a few degrees warmer than usual. It was a pretty hot day so I didnt really worry about it.

We took about an hour break. I came back and did my 5th run and it did it a little, but not much. Right at the beginning. As I shifted into second and hammered on the gas hard again. Remember, this is autocross, I want to get rolling and get it into 2nd gear as quickly as possible. Probably a 2500-4000 RPM shift point. The 6th run it was really bad, it did it through first gear, and then continued through second gear for a few seconds, and then smoothed out the rest of the run as usual. Each run was ~40 seconds long. This always happened in the first 5 seconds of each run. After that it was perfectly fine.

After that 6th run, I decided to pop the hood and check a few things. fuel pressure was way low. About 3 psi. It's normally around 6. I've noticed before that my fuel pressure varies quite a bit based on temperature, but I turned it back up to 6 psi right there in the staging lane. It was 6 psi cold the night before, because I checked it as part of my once over of the car. Fuel lines were hot to the touch, but so is everything else in the engine bay. It's just hot in there with the long tubes and no cowl.

I went to do my 7th run, and it began breaking up really badly so I just decided to abort the run and turn and park the car to see if I could catch whatever it was in the act. And as I put the clutch in to throw it in neutral it just immediately stalled and died on me. I tried to restart it (to see if it was an ignition problem) and it took a lot longer to crank than usual and I caught a quick whiff of "flooded engine" smell as it restarted. It ran fine afterwards. Fuel pressure was fine. Drove home fine after that.

But I need to solve this problem for next time, and it's never done this before except years and years ago it did something similar at the drag strip (on a very hot day) as the fuel vaporized in the lines and wreaked havoc in the carb causing a bunch of fuel to be blown out the vent tubes and into the engine. Im inclined to think this is the problem, but the main difference is that at that time I had the car turned off fora few minutes. Long enough for the fuel to depressurize in the lines and heat soak. It flooded on restart but I figured it was "okay", but as soon as I put a real load on it, it bogged down again going down the track. The difference between taht time and this time, was that time I KNEW the car had flooded and I had merely thought that I had cleared it. This time the car was running constantly, with fuel constantly under pressure. I know it never flooded. Which means that if it did "sorta" flood, it was "minor" percolation blowing small amounts of fuel in there, just enough for it pool in the checker grid in the bottom of the intake below the carb flange.

1. It's definitely heat related.
2. It SEEMS to be excessive fuel. Almost acting like a float is stuck sometimes. But it's only in certain situations. Fuel is below the sight plugs, just enough in the front that surface tension keeps it from spilling out, and a tad lower on the secondaries
3. Long tube header heat against the fuel lines in the trans tunnel? Thinking about getting some header wrap for that area.
4. Heat build up in the fuel tank due to the muffler being under it? Maybe?
5. I think fuel may be somehow puddling/percolating into the intake and sitting in there until I put some G-force loads onto it or crack open the throttle aggressively, after which it immediately tries to flood the engine. Because it seems G-force related as well. But only at the beginning of each run. We have a quick chicane at the entrance. The initial launch is okay and then a quick turn as I hammer on it, then it starts to break up a little. ThenI let off a tad and go around a hard fast corner, and that's when it really breaks up really bad. Thats probably the highest G-force load it sees at first. But what could cause fuel to puddle up in the intake ONLY when its hot?
6. One last thing that makes me think it's a flooding problem, is my new wideband sensor failed right as this started. I believe the sensor itself got raw fuel splashed on it, which from what I understand is lethal to wideband oxygen sensors. Inconveniently it also meant I had no idea whether it was running rich or lean when these issues happened.

I've been told my fuel regulator position on the fender isnt ideal and that I should put it on the intake near the carb, and that may solve the issue, but Im a little confused at how pressurized fuel in the feed line can vaporize/boil/percolate while the engine is running. Is this a crazy notion?

My plan as of now is to figure out how to relocate the regulator closer to the carb (And there's no good way to do it at the moment) and get some header wrap for the collectors. But Im not entirely sure either of those will help.

Anyone with a similar setup have any advice?



Also, despite the engine problems, I did win a trophy yesterday. The car was stupid quick and impressed a lot of people.

five7kid

iTrader: (14)

Figure anything out yet?

Some of the usual questions:

Are you using a return-type regulator? I don't think there's anything wrong with your regulator question. I have heat-tape-wrapped my fuel line, though.

Is the fuel pump the original, or a replacement? I've had them give out when they got hot. The factory TBI pumps don't seem to have a very good reputation when used in carb applications, especially when engine output is upped.

Do you have the shield in place between the muffler and the tank?

Have you considered a carb heat shield?

Damon

I don't think the position of the regulator is all that bad. I've always put mine there when converting to carb and never an issue.

Is your fuel pressure gague a liquid-filled type? If so, I wouldn't trust it. Reading low while warm/hot is something they are NOTORIOUS for.

Agreed on the carb heat shield idea. If all you have between the intake and carb is a thin single gasket, heat will get up to the carb very quickly on a hot-soak.

Fuel pump might be on the way out. Like anything electrical, they usually start to fail when hot.

Speaking of electrical.... guess what else likes to fail when it gets hot? Ignition stuff. Coil, module, pickup... all more likely to get flaky on you when hot if they're on their way out already anyway.