'62 FI Hot Idle Problem

[Rob T]

Hello everyone,

I am having a problem with my '62 fuelie at a hot idle. Based on my search of the board and archive, at first I thought it might be a percolation issue (which I definitely have during very hot weather) but I am no longer sure if that is the sole problem.

The car cold starts without any issues and idles smoothly at 2000 rpm. Once the car is warm and the idle kicks down, it idles at 900 or so with a typical lopey idle and has no noticable miss.

The problem occurs during one of two scenarios that I've noticed: after idling at 900 for 5 minutes the idles slowly begins to slow to about 500 and the car seems to almost develop a miss. The same thing happen if I am driving and come to a stop light--the car idles perfectly for a 30 seconds and then the slow down occurs. During both scenarios, the car will die after 30 seconds or a minute of the rough low idle. I can also see the ratio lever basically bouncing off the economy stop when the rough idle sets in. When I'm driving the problem temporarily fixes itself anytime I get moving again, only to resurface when I come to another extended stop. If the weather is very hot (92+), the car does not like to hot idle at all.

I originally thought it was percolation but this same thing occurred to me a couple of times this week and the weather has been in the upper 60's both times and the fuel lines felt hot, but not too hot to touch. I run pump 93 octane fuel and have run all but perhaps a trace the winter blend out of my tank.

What are your thoughts?

Thanks in advance. This board is a wealth of information.

[William C.]

Re: '62 FI Hot Idle Problem

My thoughts are the cranking signal valve is leaking, shut off the vacuum line from the CSV and see it it will hot idle than.

[Gerard F.]

Were you in Sonora this week? *NM*

[Rob T]

Re: Were you in Sonora this week?

No, that wasn't me, but by golly it could have been given the symptoms.

[Mike E.]

Re: '62 FI Hot Idle Problem

Rob--I'll be interested in the responses you get--to me, they symptoms you describe are part of being a 62 fuelie. I've had 12--11 of them driven, and only one didn't have that those characteristics. I've always chalked it up to gas quality, because the one I've had for 29 years didn't do that back in the late 70's.
The only one that never did that was the Gulf Oil racer, and I'm not sure I ever gave it the opportunity to idle for that long, nor did I use it under "normal" driving conditions that regularly. It was almost always a non-idle situation. Good luck, and as I said, I hope someone weighs in with some good ideas. I'll have to check out Clup's suggestion.

[G B.]

Modern gas and FI percolation

Over the last four months I've heard complaints about spider percolation from several '57 - '62 FI owners who never had this problem at idle before. I've been discussing this with retired engineer and '62 FI owner Gene Flanders in Hattiesburg, Mississippi. He's talked to several state officials and oil company reps. Based on Gene's research, here's what I think is happening.

The ASTM distillation specifications for gasoline are referenced by most state governments regulating fuels for public use. These specs have changed a little in the last 25 years. The biggest problem (for us old FI folks) is that gasoline can now be sold with a 50% boil-off temperature as low as 150 degrees during five months of each year. This is the temperature at which half the gasoline will vaporize at sea level pressure. Years ago it had to be at least 170 degrees year-round. The 50% boil-off allowable maximum has always been at least 235 degrees.

I'm guessing many U.S. refineries are now routinely producing gasoline near the allowable minumum 50% boil-off temp. In the past I think they made gas closer to the high end of the allowable range.

New car FI systems operate at relatively high gasoline pressures. High gas pressure raises the boiling temperature a bunch. Our old Rochester units have a nozzle pressure of only .1 psi at idle. This low pressure doesn't have an impact on gasoline boiling temps.

You can run Aviation gasoline, but it's no sure fix. It has a 50% boil-off allowable minimum around 170 degrees. I've tried using it in 100% concentration and it didn't faze the problem. Those special gasoline additives and octane boosters sold at auto parts stores don't have any significant effect either. Neither does the additive sold at swap meets that is 99% kerosine.

I've tried the one-piece plenum to baseplate gasket too. It was no help. Insulating the line from the engine fuel pump to the fuel meter made percolation worse.

The only cure I've found so far is to run 100% racing gas. I use the BP brand with a 108 octane rating, but other brands would probably work as well. I tried cutting it 50% with pump premium to save money, but the idle percolation came back.

The surprising thing to me is that the die-cast plenums used from '63 to '65 don't seem to cause spider percolation at idle.

[Jerry G.]

Re: Modern gas and FI percolation

I have a 65 Fi car. I live at around 5500 elevation and was having the same problem. I used to work automating refineries and what your saying about boiling point is right on. I "cured" the problem with racing gas. I can get away with a 50% mixture if I have to. Jerry

[Mike E.]

Re: Modern gas and FI percolation

Jerry--
I've heard over the years the suggestion of using the 57 one-piece plenum to base gaskets (vs. the 62 4-piece) to help fight percolation. My 57 FI car hasn't been prone to percolation. What do you think about that as a step in the right direction? Short step? Any value at all?
Mike

[Kevin T.]

Re: Modern gas and FI percolation

I am one of those guys that was experiencing exactly what was described regarding percolation in my 59 fuelie.

I know my FI unit is set up correctly because Jerry Bramlett recently restored it and set it up on my car. Jerry and I had been troubleshooting the problem for a while before, and he eventually came up with the aforementioned explanation about the quality of modern gasoline and the variations between winter and summer blends.

I have owned the car for about 2 1/2 years, and reside in Chicago. Most of my warm idle problems occured when I was most likely using "winter" gas. I use Shell or Mobil 93 octane gas. I recently found a number of gas stations that sell Sunoco 100 octane in the outlying area, and was prepared to try some of that. However, I have not been experiencing the percolation problems in the last two months and have not bothered to chase down the 100 octane fuel.

We have been experiencing warm summer weather in Chicago lately - but the engine has been running smoothly. It idles rather nicely at about 750-800 RPMs. As a result, I am inclined to support Jerry's explanation. Of course having said that, next time I am out with the car the engine will stall at each intersection while idling. Such is life with a vintage Rochester fuelie. I will keep you posted regarding my future experiences. - Kevin

[Michael D.]

Re: Modern gas and FI percolation

i have a 58 and a 59 fuelie and this year the idle problem has been esp bad. it doesnot even have to have hot weather. just turn the car off and let it set a couple of minutes and it doesnot want to idle. long stop lights-bad. the source of race gas here went up to 8.00/gal plus salies tax of 9.2 % so about 9.00 gal. lets talk more about aviation gas. has anybody used it where it worked? i understand you have to have an n-nuber to buy it. i posted this question about av gas a couple of weeks ago without responses . glad to see some action thanks in advance mike

[Michael D.]

Re: Modern gas and FI percolation

i have a 58 and a 59 fuelie and this year the idle problem has been esp bad. it doesnot even have to have hot weather. just turn the car off and let it set a couple of minutes and it doesnot want to idle. long stop lights-bad. the source of race gas here went up to 8.00/gal plus salies tax of 9.2 % so about 9.00 gal. lets talk more about aviation gas. has anybody used it where it worked? i understand you have to have an n-nuber to buy it. i posted this question about av gas a couple of weeks ago without responses . glad to see some action thanks in advance mike

[Larry L.]

Re: Modern gas and FI percolation

Aviation fuels:
Typically there are three grades 80, 100, 115 all are rated as lean but are specified as lean and rich.
100 low lead is 100 octain lean and 115 octain rich. - This is still to much lead.
115 high lead is 115 octain lean and 130 octain rich. - This is really high lead.
Aviation fuels are not blended as auto fuels are because they do not have the aerometics. Aerometics would boil off at low pressures above 10K feet where the pressure is about 10 psia. because of their vapor pressure.

The high lead even when mixed 50 / 50 with 94 octain Sunoco will still lead foul your plugs. 100 octain when mixed 50 / 50 with 94 octain Sunoco works well with 11:00 compression ratios. The down side of this that if you have a carberator forget about starting you car when the temperture is below 30 deg. F. Injected cars do not have to worry about low temperture evaporation.

Another problem with the mechanical injectors is that the set up for the power stop and econmoy stop is based on pressure - you should not care about pressure but are mainly concerned with air / fuel ratios. A relatively simple way to measure fuel / air ratios is to compare the thermal conductivity of C0 and C02 coming out of the tail pipe.

So now what. A long time ago Sears had a fuel air ratio meter that allowed you to stick the probe up the tail pipe and a meter on a long cable that allowed you to read the fuel air ratio while driving the car or working under the hood. I happen to have a couple - not for sale.
Use the cloth belts to clamp the sensing unit to your bumper, put the flex probe up the tail pipe and read the fuel air ratio. Set the economy stop for a ratio of 14.5:1, drive the car, at low speed put it high gear and floor it and see where the fuel air ratio goes. Adjust the power stop for a fuel air ratio of 13.4:1 and most of the problem is gone. Remember that there is no temperature measurement capability in the mechanical injection so do this on a warm day. The ratios are not as rich or as lean as could be, but close becasue there is no temperature term in the fuel demand, it is volume and not mass flow.

A good reference for all the fuel air ratios is web sites for a number of avaition fuel and auto fuel companies. How to hot rod the Small Block Chevrolet from 1970 - it may still be in print and has all the data. It applies to any engine.

Spark plugs are the other problem - AC44's were designed for all around use but you need a variable heat range plug - Get a set of AC43S extended tips and no more plug fouling - I have a set that have been in my low mile car for 20 years. Once a year I pull them, look at them, dust them off, lube the threads and back they go.

There is a simple procedure that was developed by the FI developers to set the lean (economy) stop. As soon as I find it I will post it - it is dead on with the air / fuel ratio numbers.

[Don H.]

Aviation fuel

I have ran 100LL aviation fuel in my 60 FI for over 20 years with no fuel problems. I have to purchase it in 5 gallon cans and was about $3.80 a gal 2 weeks ago. I put a kit in the unit (320) a few years ago and the fuel meter was clean as a whistle. It is also great for seldom run items as it does not go bad for a longtime. Don H.

[G B.]

What works.

I haven't found any single step (other than running 100% racing gas) that will cure spider percolation at idle caused by modern gas with a low boiling point. The BP/Amoco 108 octane racing gas typically has a 50% boil-off temp of 210 degrees. You will still have heat soak percolation for the first 15 seconds after a hot re-start, but once the engine is running smoothly it will idle forever with racing gas.

Here is an 8-step plan that MIGHT work to allow you to run modern pump gas with a low boiling point in a '57 - '62 FI unit without having idle percolation:
1) Install an electric fuel pump near the tank and by-pass the engine mechanical pump. The mechanical pump pre-heats the gas as it passes through it.
2) Reroute the fuel meter gas inlet line and filter far away from all engine heat sources.
3) Install the thick, one-piece plenum to baseplate gasket.
4) Remove the bracket that bolts the spider to the plenum bottom. Make sure the spider lines and hub do not contact the plenum anywhere.
5) Position a piece of heat insulation mat beneath the fuel meter. Also sandwich some of the stuff under the brackets that attach the fuel meter to the plenum.
6) Remove the hood and store it in your attic. Also, for '58 - '62 models, seal the open area between the top of the radiator and the nose fiberglass.
7) Use a heavy duty aluminum radiator, 160 degree engine thermostat, and an 18" diameter GM 7-blade clutch fan.

And most importantly, #8: Consider running any stop light that lasts longer than 60 seconds.

[Dennis C.]

Jerry, Good list of cures...

...My experience would put your #6 (storing hood in attic) as the single most effective way to fight the problem (other than race gas, of course)...