Engine Surging in slow traffick

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.

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

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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.

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Lay hands on a non-contact temperature measurer.

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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.

Well, I am finally back to this. I added a thermal wrapping ( here ) to my fuel line close to the engine. Last weekend I vapor locked. Engine stopped and I had to wait until the engine cooled.

I have not had this happen before. But, based on earlier comments in this thread, this is just the worst end of my problem, most likely.

I am tempted to get an electric fuel pump, but earlier in the thread, kcress said that doesn't always work.

I may break down now that I am experiencing this and get one of those temp sensors.

By the way, anyone know the limit on temp in the gas line at which vapor lock will occur? I presume not, but thought I'd ask.

Just thought i should ask,,,how long have you had this problem with surging??
Has it alway's happend?? since you owned/built the car?? or is a recently occuring problem??
Of course taking into acount the same time of year, driving condition's and weather and all that.
If it's only a recently occuring problem:
There is a condition called percolation,,which can happen as a result of air bubbles in the fuel,,,usualy caused by a small crack in the fuel pump diaphram,,,which allow's air to be mixed with fuel prior to delivery to the intake system,, with this condition,,the float loses boyancy and drops momentarily.
The same principal applies to any object which needs to float on liquid,,airiation will cause loss of boyancy.
This condition can also occur due to extreem heat conditions also.

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

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If I understand it right you're running a Holley carb, correct?

The Holley manifold referenced power valve throws me.

The power valve is referenced to the intake manifold that sits on the heads under the blower?

Or are you calling the adapter between carb and blower the manifold?

I'm wondering as well what you have to do to reference vacuum for the power valve from an external source.


Perhaps referencing the power valve to an outside source is the correct thing to do, but the Holley's I see on top of a blower operated the same as they do on a naturally aspirated engine.

I wonder if it's something as simple as the wrong power valve selection for the cam intake combo you have?

That happens a lot with naturally aspirated engines where a guy runs a big cam with a stock power valve and the power valve opens at idle & low speed dumping excess fuel into the intake.

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Originally posted by DennyW
:) Yes, you have to match the power valve to the intake vacuum, plus, you have to make sure you have a hole that goes to the vacuum source from way up there. ;)

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Ok, but I'm still not understanding.

You source the vacuum from the intake under the blower, right?

And then you have to drill the Holley carb so you can set up vacuum to the power valve?

It sounds reasonable and a little tough to do.

Seems too that just plopping the Holley on the adapter plate on top of the blower would work ok.
That would be under a vacuum at low speeds and once the engine/blower were spun up, the vacuum at the adapter plate would drop close to zero and would allow the power valve to open.

The advantage I can see with an intake sourced vacuum to the carb would be that the power valve would probably come in quicker when the intake vacuum rolls over to pressure when rpm's come up.

Thanks Denny.

I think I have it now.

Dang . . . I gotta get me one of them blower thingy's one of these days....

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Just thought i should ask,,,how long have you had this problem with surging??

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This has been going on for the past couple years. The 18 years before that I have not had the problem.

In answer to other questions:

I have a Holley 80273S carburetor. That is a carb that is designed with the vacuum hookup to provide the manifold referencing. So, as DennyW says, the vacuum port is already there.

However, if I understand DennyW's replyl correctly, he is saying to put the vacuum to an adapter below teh carb, but above the blower. That is inaccurate. The power valve needs to know what the vacuum is at the manifold. That is what determines how it works. That is what is meant by 'manifold referenced power valve'.

Instructions on that are here on the Holley site.

With regard to which power valve, I have had multiple conversations with Holley tech on this. They say to take idle vacuum and divide by 2. So, my 9.5" vacuum gives me a 45 power valve. I have tried lower and higher. I seem to constantly be over rich, no matter what I try. It doesn't seem too bad right now on richness, but a little rich. I do have the 45 in it now.

Earlier posts indicated that possibly my surging was a 'minor vapor lock'. And, given that I had a hard vapor lock (no fuel getting to engine at all) last weekend, that lends credence to that theory in my mind.

One thing I have changed (don't remember exactly when): I hooked my electic radiator fan that is on the front of the radiator to a temp unit that is supposed to turn it on. Problem is, the radiator has the place for that at the bottom, instead at the top. So it comes on rather late. I seem to have problems when my engine gets to 200, which isn't all that hot. Although it gets quite hot under my hood. (I do have a fan on the front of the engine as well. I had hoped to pull the electric one off the front, but now, I need to get it going all the time I guess.)

So, my immediate problem is "How do I best cure vapor lock?"

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Somebody tell Demon I went outside the box, and got shot down

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Sorry. Didn't mean to come of as 'shooting you down'. I apologize if I was insensitive in the way I posted.

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it doesn't take long to go from neg to pos pressure in the intake

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That is exactly right. And I think the point is that when it goes pos at teh manifold, they want the carb to know that, otherwise it will dump the wrong amoutn of gas.

Interestingly I ran that blower from 1985 to 1999 w/o a manifold referenced power valve. It always ran way too rich, and a call to Holley pointed me to this manifold referenced power valve carb. The explanation was that teh carb had no idea that the pressure was pos instead of neg, and dumped too much gas.

Interstingly, I still fight it being somewhat rich, but not near as much.

By the way, wouldn't a surge due to float level being too high happen all the time? It only happens under the circumstances I described at the beginning of this post.

I do feel pretty comfortable with the float level. I actually have it just a smidgen lower than the docs say to do it.

I'm still hoping for some definitive insight on fixing the vapor lock. Should I open a new post titled 'Vapor Lock' for that info?



:)

I just went out and paid more attention to the gas line, looking for places it may be getting hot.

I found that while I am running it on the outside of the frame, I also have my tranny cooler near it at one point, so that may be another place to insulate.

But, I also realized that back when I built this thing, I used copper for the fuel line. I wonder if I replaced that with steel, if it would help.

Thanks

I fixed the "here" link. I should have checked it.

I did another past, and pasted that there too.

I'll open a new thread on Vapor Lock, and on the copper fuel line.