Thread: Engine Surging in slow traffick

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My engine is a 350 Chevy engine, and it has a B&M Street Blower (5lbs boost) with a Holley manifold referenced power valve.

The problem I am having is that if I am in stopped traffic for a period of time (i.e. a rod run consisting of 5000 participants, and stopped or moving 1-2mph for 5 minutes)...when traffic then picks up the engine surges. For example, it's as if I were pushing the pedal, letting up, pushing, letting up. This happens until I get moving to 30 mph for a few seconds then clears up.

One thing I am sure of is that it is getting quite hot under the hood...and when I get moving the air flow relieves that heat. I expect it is heat related.

That being said, I'm not sure in what way.

Any thoughts?

I would say you are getting vapor lock. The fuel line under your vehicle near the exhaust is getting hot enough that the fuel vaporizes in it and can't be pumped. This causes your carb fuel levels to drop radically which then causes large changes in your fuel delivery to the engine. If you were in motion the air passing under the vehicle would be enough to prevent this normally. Then as you actually accelerate these areas of vapor show up at the carb spurt, bubble, spurt, bubble, spurt, causing exactly the symptoms you are reporting. Soon the air cools the line back down and away you go.

To fix this you can:

1) Try putting an electric fuel pump in/near your tank. This helps but not always.

2) Put heat shields on your exhaust pipe ala like what's on motorcycles to prevent leg burning.

3) Put a heat shield, sheetmetal, next to the to fuel line.

4) Re-route the fuel line.

Good luck.

Thanks. I was wondering about vapor lock, although I thought that resulted in teh engine not running. Maybe it's just not severe enough.

I'll try some of the suggestions.

Also, would overly hot air going into the Carb be of any significance?

ewingr, Yes, vapor lock should actually maybe, be called vapor clog.. It can be very subtle and annoying. For instance on a cooler day doing the exact same thing you might not have a problem. Or if it was a really cool night and the fuel in the tank is 10 degrees cooler, or you paraded along 3MPH faster, etc, etc. The usual solution is to execute the repair and discover you no longer have the problem.

No the hot air would not cause the problem you describe. Further when you accerate away you are running truely mind boggling amouts of air thru your system. Which means any local HOT air is consumed in a few 100 milliseconds. Your engine when consuming 200 cubic feet per minute, (about half throttle), would use all the air in your entire engine space, carb, air cleaner, radiator, grill, free space around the engine, in less than one second. SO if your problem lasts more than one second this isn't it.

Floats set too high?

I'm sure it's not the floats. I've checked that, and it only happens in stopped/very slow traffic...and I have to be there for a while.

I experienced the too high float already and fixed that some time ago

I'm thinking of first putting some heat shielding on the fuel line. Something like this .

I wonder if vapor lock can happen after the mechanical fuel pump. Any idea about that? (I'm trying to decide if I would need to put that wrap on up to the carb. I'm hoping not.

Are u using a good heat gauge, do u know what temp u actually reach in traffic?

ewingr, That stuff looks like it will do the job.. Seems kinda pricey tho.

Ives' comment gave me an idea. Lay hands on a non-contact temperature measurer. One of those point-the-laser-spot-read- the-temp guns. Warm up your ride. Idle around your street, parade wise, for 10-15 minutes. Then dart underneath with the temp gun and scan the entire fuel line.. This would be VERY telling!

Remember the temp gun sees the light of warm objects so as you use it imagine everything giving off light that signifies it's temp, and use the gun appropriately. Another words don't point it at a fuel line that has the exhaust pipe behind it as the light from the exhaust line will trump the fuel line and your reading WILL NOT be telling you the temp of the fuel line.

You may find that you only have a one or two foot section of fuel line that needs insulation or shielding.

Between the fuel pump and the carb?? Yeah, you can get some boiling fuel but it is a lot less likely to be the problem.

Are u using a good heat gauge, do u know what temp u actually reach in traffic

I had changed my heat gauge last year, and it reads the same as the one I replaced. So, I am thinking it is probably right.

Lay hands on a non-contact temperature measurer.

Interesting idea. I doubt I can come across one. And, it looks like the least expensive ones are close to $300. I think the Cool Tube will be less to start with . I'll put it on the fuel lines that are nearer the headers.

Well.... Ohhhh kay... but here are a bunch under a hundred bucks.

These have other uses...

I use mine to check the temperature of my aquarium, the chili I was cooking yesterday, room temperature, my oven temperature.
You can use these to find which cylinder is missfiring. The one with the 20F colder header pipe....

http://www.techinstrument.com/acatal...emometers.html

http://www.toolsource.com/ost//produ...GDTVL90HBJFFME

http://www.calright.com/_coreModules...ctMasterID=188

http://www.nottheusualstuff.com/cata...products_id=50

Thanks! I must have used different search terms. I found several in the over $200 range, and didn't look much further. I thought if there was something sub $100 it probably wouldn't be of good quality (accurate).

Are you using a less expensive one?

Well don't laugh... I have two! One really expensive one $350 and one really cheap one. The least expensive one $60=$70 one.

The inexpensive one is simpler and works just great! I use it the most infact. The major difference is the distance you can get an accurate read. My expensive one will let you read a temp from about ten feet seeing a spot 6" in dia. Where as the less expensive one will see a 6" dia spot about two feet away. The optics for the infrared light are expensive, I guess.

95% of the time you will be closer than 2 feet when taking a temp anyway!

Excuse me but I need to check the temp of my french toast! And taste!

Yeah, I just bought an infrared thermometer at Sears for about $60 (the "Craftsman Club" sale strikes again). It has been really useful, as I try to figure out the overheating in my car. Also works to check for hot spots in the data center...

Good luck.

One other option you can try to get temperature info less expensively is by using thermometers. You could try to use a common baking thermometer taped to whatever you want to check, but more industrial-use versions exist that look more conventional and read temp only at the end. That long stalk on the oven units can foul your readings unless insulated.

The other option: do a Google search for "temperature strips". Tempil and others make one-way indicators that change color permanently once a certain temp has been reached. Since you can't know what temp you need to indicate, they also fortunately make strips with an range of temps on them like a thermometer. You could buy a sheet full and stick them onto several places. Start up and run normally, jump out and note strip readings, and then just sit awhile and read again looking for differences in areas affecting fuel.

Your only challenge with this last one is to find a temp range that fits what you want, and finding a source that will sell you just a small quantity. Businesses doing retort food processing tend to use them a lot, and you'll likely need more than just one freebie sample since they are one use only.