[amcmike]

Below is the original question posted by mechanic17 in a different thread.

mechanic17
Member

Join Date: Dec 2007
Posts: 89

No intent to hijack here, I just figure Cliff has the attention of some of the FI gurus here, and I have what I think may be a simple question for you about stock TBI (of the 1993 vintage).

A mild flat tappet cam 218/224 @ .050, .462 lift should not cause my previously healthy Chevy 305 to idle at 8 inches of vacuum, should it? I'm assuming I need a FI remap or whatever you Buck Rogers types call it, but is there some other check/solution?

Car actually runs good, sounds good, but not getting enough vac for my brakes and others have reported this combo idling at 14 ". Oh, and I put a Perf RPM intake with TBI adapter and have triple checked, no leaks.

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I’m more familiar with port fuel systems than TBI, but operation is somewhat similar, so I’ll take a crack at it.

First off, the cam you chose with that compression and engine displacement is not really mild (not wild either, but somewhere in between). Second, the effect at idle depends on the overlap, so the LSA would also be a big factor. Injected engines like less overlap, so typically LSAs of 114 or more work best. Now that you’ve changed the flow characteristics cylinder filling at idle, part throttle, and wide open throttle (WOT) have changed. Therefore fuel and timing requirements have also been affected. But there are some things you can try without spending money.

Some background.
After warm-up, your engine will go into closed-loop mode. The ECU will use the oxygen sensor readings to achieve a stoichiometric air/fuel ratio (14.7). When you go WOT, you will switch to open loop. At that point the ECU is commanding fuel based on pre-determined settings that are not real-time compensating.

So, when the fuel requirements are affected in closed loop operation, the ECU will try to correct by added or subtracting fuel until it reaches stoich. There is, however an authority limit as to how much the ECU can adjust. So it may try to a point, and then that’s the best you can get. Now as I said, at WOT, there are pre-determined settings since there is no feedback, however this is not always 100% true. With some, if the ECU senses an afr shift at closed loop, it may apply some of that compensation at WOT.

With your new cam, the extra air will have leaned the afr at WOT. However, most factory systems run more rich than necessary for peak power because:
1) Each engine is going to be a little different because of tolerances.
2) Conditions under which the vehicle will be driven will vary from region to region.
3) Helps to cool the catalytic convertor
Basically, they error on the rich side because it’s safer. So you might not necessarily need to compensate at WOT.

At idle, you may not have the right amount of fuel and/or timing to get a stable idle. If it’s mostly fuel, there are some things you can try. Again, if the ECU is already being trying to compensate the fuel side, it may not have enough authority. You can try to adjust by adjusting the fuel pressure. (You could also use different injectors, but until you know whether it wants more or less fuel it doesn’t make sense to go this route, plus you’re spending money). It’s not as precise as a real tune, but it depending on which way you need to go, it might be enough to make it run better.

“Do I need an adjustable fuel pressure regulator?” Maybe not. Some of the Delphi can be converted to an adjustable. There is a spring inside the regulator and the preload determines the set pressure. Many of the Delphi regulators actually have a star patterned screw in the back. The problem is, to make them tamper-proof, they add a little nub in the center, so a normal star bit will not work. But you could grind that portion out, and then you can change it. Caution: Always have a pressure gage and record fuel pressure with the vacuum connection (if it exists) disconnected before making changes.

Turning the screw clockwise increases spring load, which increases pressure, which enrichens the mixture. Turn counterclockwise has the opposite effect. Note a lot of the early systems have a vacuum biased regulator, so a tube connected to manifold vacuum drops pressure as vacuum increases. So at WOT, it’s like having the vacuum line disconnected. But at idle you have most or all vacuum compensation. This is to keep the effective pressure across the injectors the same regardless of load. The gage pressure you read will vary, but the drop across the injectors (because you have manifold pressure acting against the fuel pressure) remains the same.

Try small adjustments (I’d go rich first) reconnecting the vacuum line, and see if it clears it up. Actually a quick check to see if it likes richer would be to disconnect the line and let it idle this way. Because there is no vacuum, the gage pressure will increase, allowing more fuel.

Once you evaluate it’s affects you have a couple choices. If it’s not enough, you may need a tune, because the engine will want more timing. If you try to adjust timing by resetting the initial, the ECU will start pulling timing out until it gets back to it’s factory setting. If you overcompensate to fool the ECU, it will go into limphome mode and run terrible.

If reducing fuel pressure improves the situation, you will need a tune. Because in reducing the pressure at idle you’ve also leaned out the WOT. If increasing it helps, then you could probably get away with running the higher pressure. So once you’ve done all your testing at idle, take it for a test drive. If everything runs good, you should be fine. Record your new pressure for future reference.

My only concern is you've stated others claim to not have issues with this combination. Which makes me question what is different about your engine. Did they also do the intake swap and adapter just like you? Larger port runners and different lengths are also going to affect cylinder filling at idle.

[mechanic17]

Wow, man, thanks a million for taking the time. It helps. I'm fairly intelligent and am slowly making my way out of the stone age, but I've resisted FI except for my perfectly running and untouched LT1 Roadmaster. And this Caddy that the 305 lives in, just a fun project with a tiny budget.

The "others reporting 14 inches vacuum" are running carburetors, so I am assuming the TBI/tune are the difference and I'm now far from stoich by adding this cam. The motor gets plenty of intake/exhaust flow with the 350 injectors, Edlebrock intake, stock HO heads, and headers (I did include the O2 sensor, though after the collector, so I'm wondering if it even gets hot enough to go into closed loop at all.)

I think I've read before about that fuel pressure mod and I'll try it. Best guess, the car came with a 350 (sorry I didn't mention that in original post), so too much pressure, or flow, if anything? (Was foolishly hoping the bigger cam and 350 injectors would kinda compensate for each other.)

As you say, the car does go good at WOT (which makes the brake problem very scary LOL). Doesn't pop or blow white smoke or smell rich that I can tell. Idles like a Harley, though, and "hunts" (as the computer tries to compensate for the cam overlap?)

Hey, since you mention timing, what about going to a standard vac advance HEI to eliminate the computer control there? (I've read in the Impala SS forums that this is a simple and effective swap, and I have the part already.)

One more question: I'd ultimately be willing to invest in a "meter" or better yet a laptop cable and software, if I knew one with the flexibility to work on OBD1 (this car) and OBD2 (everything else) and/or, dare I say it, my '09 5.3l Silverado. Any recommendations?

Don't bother with an extended reply--you've already done alot, and I appreciate it. Cliff's thread, anyway.

[PunchT37]

I don`t remember TBI having an adjustable fuel pressure regulator.
I put a Comp 260/268, that`s 206/218 @ 050, on 112 lsa in a `88 305 TBI motor. Ran ok but had a 45 rich code that stayed until I retarded the cam 4 degrees.
I believe even that cam had too much overlap for that early efi system.
Problem with those TBI systems is, they are inflexable to changes made. They operate on a speed density system[map sensor] and not a mass air system. The programing and the hardware they used is also a problem. Most mass systems will "absorb" a cam like that with no problem these days.
The 305 cubes is agrivating the problem compared to a 350.
Might want to verify the cam timing[ICL] and have the prom or mem-cal changed out to one that can handle the lower vacumn of the new cam.

[mechanic17]

I degreed the cam, so I'm fairly certain it's in right. If I have to open her up again, it's likely I'll just put in the original 350 roller cam, which should be plenty mild enough for the ECU. But I'd rather not, as the car sounds very badass this way.

I don't understand how putting a bigger cam in (flowing more air) would cause a rich condition. Is it the increased overlap (residual exhaust contaminating the incoming intake charge?) at idle?

I will try what's already been suggested. What if, as a further step, I install a cheap adjustable fuel regulator and pop on a borrowed carb at 6 psi or so (I believe the TBI system probably uses 15psi or so?) for testing purposes. Will I have to hook up the return line somehow, or can I just plug it? If the vacuum jumps up, what have I learned?

[amcmike]

Originally Posted by mechanic17

I don't understand how putting a bigger cam in (flowing more air) would cause a rich condition. Is it the increased overlap (residual exhaust contaminating the incoming intake charge?) at idle?

Increased overlap will dilute the charge as well as bleed off pressure at idle. A second factor is the 350 injectors, which is probably OK for the WOT, but overkill at idle. Add to the mix that you're O2 sensor might not be warmed up enough to function properly (which means it will either cause the system to run richer, or send it into open loop altogether, and you're proabably way rich at idle. The O2 sensor should be as close to the collector as possible, if not installed in it directly.

[mechanic17]

I'm assuming running rich would also mean it uses a lot of gas, even just sitting there idling in the driveway? Seems like a stupid question, but I want to be sure. . .

[mechanic17]

So can anyone recommend a good OBD1 plug-in laptop interface to mess around with A/F ratio, etc.? Remembering that I'm new to all this remapping stuff. If I have to spend several hundred, I'd rather do it on a tool I can reuse, rather than just a burned "hyperchip" or something like that.

[amcmike]

Rather than spending money up front with remapping, I'd get a good wideband afr setup (Innovate Motorsports has one of the most accurate and fast response time for the price) and see where you are at. Again, if your mapping in scewed all in the same direction, you can sometimes cheat a little with fuel pressure during open loop, then simply let the ECU relearn for closed loop operation.

If you do decide after that to start tweaking the cal, here is a good website for the ECU you are probably using:
http://www.diy-efi.org/
To be honest, I haven't been there in a few years, but they used to have lots of info on the GM TBI setups including how to burn your own chips.

[PunchT37]

Originally Posted by mechanic17

I don't understand how putting a bigger cam in (flowing more air) would cause a rich condition. Is it the increased overlap (residual exhaust contaminating the incoming intake charge?) at idle?

On a engine that uses a map sensor, the lower vacumn tricks the computer to think it`s under a load, so it fattens up the mixture.

[mechanic17]

I followed AMCMike's suggestions to modify the throttle body and the engine liked the leaner mixture. Idle has stopped hunting and even the "Check Engine" light goes out for several minutes at a time. I got the exhaust tied in and the 02 sensor seems to be sending data now. Idles at a solid 13" vacuum which unfortunately is not enough for my '93 Cadillac Fleetwood brakes yet. But running much improved! Makes good power with the cam and has a idle that hints something's up, even through the caddy muffler.

Thanks so far for the help everybody. Is there anything ELSE I can do before I commit to a computer tune? I think it's still running rich at idle and gas mileage is way down yet. Plus I can't really drive it without brakes. Remember the car has headers, an Edelbrock RPM w/adapter plate, and a 350 throttle body/injectors, and the EGR and air pump have been eliminated.