Ops check: Fuel Injection fast idle

[Jim L.]

There is no specification for the fast idle speed of a cold started fuel injected engine. During ops check it is inappropriate to deduct for a cold FI engine which doesn't idle "fast enough".

I mention this because I've been contacted by a fuelie owner who is worried about this and wanted me to walk him through how to speed up the idle on his engine when cold. Apparently he's had some problems in the past during ops check.

The only specification for FI fast idle is for a fully warmed up engine. (See the charts below.) Prior to the engine being fully warmed, the idle speed can and will be pretty much anything less than the specified hot engine fast idle speed.

So if fast idle speed is to be verified, do this: Warm up the engine thoroughly. Crack the throttle while manually rotating and holding the fast idle cam into position. Release the throttle and measure the engine RPM.

Here are excerpts from ST-12 and the '63 shop manual:
c1-hot-idle.jpg
c2-hot-idle.jpg

[Dave S.]

Jim,
Is it a C1 or C2 car.?? Year.???

[Jim L.]

Jim,
Is it a C1 or C2 car.?? Year.???

Dave, I believe he has a C1.

[Duke W.]

For carbureted '63 engines there is a very simple procedure in the 1963 Corvette Shop Manual that calls for setting the fast idle speed with the engine fully warmed up. With the engine fully warmed up, but stopped, open the throttle slightly and manually close the choke valve, then release it. This should allow the choke valve to fully relax to the full open position, but should leave the fast idle cam with the notch lined up with the fast idle adjustment screw. If not, bend the fast idle cam link so the fast idle screw lines up with the notch on the fast idle cam.

Without touching the throttle, start the engine, let the idle stabilize and set it to specification with the fast idle screw adjustment, which is NOT the curb idle speed adjustment screw. For '63 carbureted engines the specified fast idle speed (HOT) is 1750.

As a general rule on a cold start in mild ambient temperatures the fast idle speed will stabilize at 200-300 revs SLOWER than what it was set at hot. On my 340 HP SWC it's about 1500. Since the '63 FI recommended hot fast idle speed is... (I'll get to that), it should stabilize at something between 200 and 300 revs slower than what it was set at hot within a few seconds of firing in mild ambient temperatures.

The '63 Corvette Shop Manual has a very clear procedure for setting fast idle speed on the FI engine beginning on page 6M-38, however, there is a discrepancy. The text says to set it at 2200 hot, but the specs section (shown in post #1 ) says 2000 (hot). Go figure! I expect service manuals for other years have similar procedures for setting fast idle on all engines including FI.

It's been my experience in Ops Check and PVs that a lot of cars don't have the fast idle speed set properly according to the "book", and subsequently often don't obtain a proper cold fast idle (200-300 less than set hot), but will usually squeak by. I think the reason is guys don't understand how to set fast idle despite it being clearly explained step by step in the service manuals and understanding that the typical cold fast idle speed after the engine fires will be 200-300 revs less than what it was set at HOT!

So why did Chevrolet specify setting the fast idle speed with the engine hot. Probably because it was easier to do. If you took a car to a shop for a tuneup they would generally let the engine cool down before working on it and by the time the points, plugs, and condenser were changed along with other checks and possible parts changes they would start the engine, connect an "engine analyzer" to check the secondary wave forms, set dwell, blip the throttle to drop off the fast idle cam and, check/adjust the timing at around normal curb idle. By this time the engine was at or close to full operating temperature, so the final steps were to go through the curb idle speed and mixture adjustment procedure followed by checking the fast idle speed (engine hot) and adjusting as necessary. At least this was the way a good mechanic should do a "tune-up" on these early post-war vintage cars.

Another problem is the hot air choke system, like on AFBs and some (but I'm not sure all) year FI engines. The choke vacuum break pistons inside the thermostatic spring housing tend to get dirty and sticky, which can prevent the choke valve from completely closing, which means the fast idle cam does not set properly and can result in knarly cold starts. Those early oil-wetted air filter elements certainly were no help due to poor filtering quality and allowing oil to pass through the hot air choke system eventually making the vacuum break piston sticky.

I also think that some fast idle speed specs (hot) like 2000 or more are faster than necessary, but they may have been speced that high to achieve acceptable cold start and drive-off at 20 below zero F winter temperatures. If you only drive in mild weather most engines can probably get away with lower fast idle speeds, but for ops check and PV set it by the book.

Something else I noticed at the Vegas (2018) and Palm Springs (2020) nationals was more than the usual number of "knarly" cold starts. Even though ops check was started before dawn, the overnight low temperature of about 90F wouldn't allow some chokes to fully set, even if perfectly clean and perfectly adjusted, which meant the fast idle was barely enough. It was a delicate balance between a barely acceptable cold start and flooding the engine.

Duke

[Jim L.]

In my opinion it would be a mistake for a judge to determine ops pass/fail based on a "rule of thumb" amount by which FI fast idle of a cold engine is less than the specified hot-engine fast idle. This is particularly true in regard to '61 and older FI units which have extremely primitive cold enrichment mechanisms.

A stopped clock is correct twice a day, right? Well, for the particular case of '61 and older FI, the cold enrichment mechanism provides the correct air/fuel ratio needed by a cold engine for only one brief moment during warm-up. It's not even as accurate as a stopped clock.

As odd as it may seem, '61 and older FI engines, while on fast idle, will often run faster with cold enrichment manually turned off! Yet, they are behaving exactly as they should, given the nature of the cold enrichment mechanism with which they are burdened.

'62 and later FI units have cold enrichment mechanisms which represent improvements over those of prior years. Even so, they don't precisely track engine warm-up and they, too, can interfere with fast idle speeds.

The only accurate, consistent way to determine pass/fail is to compare hot-engine fast idle to the published specs. If I were having my car judged and the ops checker wanted to use any other criterion, I would protest.

[Dennis C.]

For what it may be worth, when I do ops on a C1 FI, I look for noticable drop in rpm once warmed up. Sometimes, I base it on sound alone and never look at the tach.

[Joseph S.]

Jim, I'll try to answer this as a Past 63-64 Team leader. First, and most important, is that we look to make sure that the choke is actually setting and operating properly. We would expect to see the choke open at first. Then on a cold engine, after a stab at the throttle, expect to see the choke close. At this point the engine can be started. We would then expect to see a higher than "Hot idle" RPM and the engine maintain idling. This RPM level should then climb as the engine warms. If you look at any of the Judging Manuals there is usually an acceptable range for high idle. If my memory is correct it is 1600 to 2200 for 63 FI cars.
So that being said, as long as the engine has a notable high idle and operates as it should, it will be accepted. I don't believe there is a judge that would be expecting the engine to achieve "spot on" idle at whatever the GM spec is for that year and model.
Common sense as to "does the choke operate as intended" would be the goal.

Regards,

Joe

[Jim L.]

Joe, thanks for posting.

I think we are pretty much on the same page..... don't kill the owner because the cold-engine FI fast idle doesn't meet some subjective minimum RPM.

Does this happen? Apparently so. Here is an excerpt from email I received from an owner concerned about this very issue:

The ncrs guys have it way wrong since they say it is what the rpms should be at cold start

This individual has clearly been frustrated by the experience(s) he's had.

An NCRS goal is to present the cars as they would have been upon leaving the factory. Achieving the specified hot-engine FI fast idle ( on all FI units after 1957) involves tweaking a simple adjustment that even an FI-phobic owner can do. Then, during ops, it's a completely objective call..... does the hot engine idle at the correct RPM? It becomes like deciding whether a headlight works or not. It's yes or no.

It's my opinion that this is the only procedure which should be followed.

To your point about the '63 - '64 judging manual.... if it does specify a minimum cold engine RPM, do you know the source of that information? None of the GM literature in my collection calls for anything other than a hot-engine RPM measurement for any FI model.

FYI, here is what the '58 - '60 judging manual has to say:

img-1952.jpg

If it is not the intent of our club to verify FI fast idle according to published GM specifications, then the above criterion should be the alternative for all FI engines. In any event, no ops judge should be allowed to arbitrarily decide a cold FI engine doesn't achieve a high enough fast idle.

Respectfully submitted,

Jim