PeteZ28

When going to an aftermarket carb on an L69 and disconnecting ECM what happens to the in tank electric fuel pump? The factory service manual doesn't show an ECM connection to this pump.

Also, I realize the general consensus is that when going to an aftermarket carb, a non-computer distributor is needed also. Though, the service manual description of the TPS sensor doesn't say anything about it controlling ignition timing. Seems like it may be possible to put a resistor across the MC-sol leads and plug in an extra TPS without even having to actuate the TPS, and continue to let the ECM control the stock distributor. Not sure about this but I didn't find anything in the manual that indicated the TPS effected ignition timing. Anyone ever try this?

thanks,
Pete

92 BBC Z

The pump will still work without the computer, but your gonna have to reduce that pumps pressure to a more carb friendly 7-9psi.

camaronewbie

Carb friendly is more like 4-6psi!

The computer-controlled distributor has to be replaced with a vacuum advanced distributor - there's no way around that. the factory distributor is not controlled by anything fuel related, it's controlled by the Electronic Spark Control Module, which is fed info by the knock sensor in the pass side block. And it all works together - not having input from the sensors (that would be missing after a carb swap) renders the ECM basically useless, thus it can't feed any instruction to the ESC module, etc.

Trust us - after all these years that TGO has been here, and all these thousands of folks modifying these cars - if there were a way to use the factory distributor and save everyone that $125 for a vac adv distributor, it would be posted all over this board.

PeteZ28

You may very well be right. Just what I found in the factory service manual at this point. The manual could be wrong (couldn't find anywhere in the manual where it mentioned the L69 having an in-tank electric pump). Although, I think you may have misunderstood what I suggested: All sensors would be connected and all electronics active (except MC solenoid) but the TPS would not be actuated. Five7kid did this but he mounted the TPS so that it would be actuated by the accel pump on a Holley (with a 'load' on the MCS circuit). I'm suggesting the ECM may not use the TPS to control ignition timing and so it may not be necessary to fabricate a linkage to actuate the TPS with an aftermarket carb. What I'm asking is if anyone has tried this without actuating the TPS?

thanks for the input
Pete

five7kid

iTrader: (14)

Since the L69 is carb'd, and uses an in-tank electric pump, why would anything have to change with regard to fuel pressure control when going to an aftermarket carb? Neither the CC carb nor ECM control fuel pressure.

92 BBC Z

My initial reply was based on PeteZ28 saying he had an in-take pump. I assumed(shouldnt have) that the engine was fuel injected, without realizing that the L69 was a carb'd engine. A fuel injection in-take pump will work on a carb'd application but the pressure must be reduced, I know, I've done it. But if he has a factory L69, doesnt it have a mechanical pump already set for a carb. Buy the way, my 1030cfm JET Performance, dble pmper, mech sec, requires more than 6psi. Sorry for the confusion.

naf

iTrader: (7)

Why not check for yourself? Observe the timing with a light as you rev the motor then disconnect the TPS and make the same observations.

This will only show you part of the story though as you won't be able to observe the timing under low load, cruise conditions - when the vacuum would advance a mech dist. Of course if you lose EGR you may want to lose some of this advance to prevent pinging.

A scanner may help on the L69 although connecting it disables some of the timing functions on later LG4s. I can't see my timing advance on my 85 or 87 with one.

PeteZ28

Yes, it is an L69 car, so fuel pressure is somewhat stock. Actually when I replaced the mechanical pump (new from dealer original pump), fuel pressure was too high. I assumed there is some variability in them (or something) and added a regulator.

EGR's been gone a long time, even with the stock 305. Now the engine is a alum. headed 350 with zz4 cam and intake. Compression is mild at 9:1. I was thinking a mech dist could give me more aggressive timing to compensate some for the somewhat low CR. Though I'm real curious now to see if the computer system could be retained to control the timing with an aftermarket carb without the extra work of fabricating a linkage/fixture to actuate the TPS like Five7kid did. I think I will give it a try after installing the new carburetor. Trying it with the ccc qjet would likely cause erratic running and a lean condition. I'll let you know how it turns out

Take care
Pete

naf

iTrader: (7)

Why do you think the ccc-qjet will cause a lean condition and/or erratic running? Mine seems to work OK with iron vortecs and an XE262.

The L69 should have more aggressive timing than the standard LG4 ECM.

Be aware that the ICM in the dist will give a rudimentary timing advance even without the ECM connected. You can check it with a timing light and you'll see an incremental jump in timing somewhere around 1000-1200 rpms. Don't let this 'limp home' feature confuse you into believing the ECM is still controlling anything.

five7kid

iTrader: (14)

And, you can't make up for a lack of compression by advancing the timing.

PeteZ28

I meant the qjet would be erratic with a dummy TPS connected (not that it's extremely smooth with it connected). I tried this today and it did run slightly erratic but not bad. It was difficult to verify if the timing stayed the same for an actuated vs non-actuated TPS. It appeared to, but it's hard to really see the timing mark when it advances with rpm. A 'dial-in'? timing light is needed to really correctly acquire timing vs rpm data to make this comparison. It would be interesting if someone else could try this.

L69 timing is more aggressive than LG4, but how meaningful is that if we don't have numbers for either.

A quicker advance curve can be used with lower compression ratio and I think help low and mid-range power and gas mileage vs the factory timing. In general from a lot of reading of the Corvette forum C3 folks, being able to bring in ignition advance sooner (to a point) really helps low end torque. And they also gave me the impression that factory emission timing curves are lazy and hurt low end power and mileage. Yes, I do wish I had gone with closer to 10:1 vs the 9:1 I have.

dmwracing42

iTrader: (3)

Sorry to dig up an old thread. But can anyone confirm that the in tank fuel pump from an '84 will work without the ECM? I've heard differing opinions, and if I could avoid pulling the gas tank that would be fantastic.

naf

iTrader: (7)

Try it and see. Disconnect the power wire to the ECM that runs along the inner right fenderwell from the battery and see if the pump spins up.

The pump relay should be on the firewall, left side near the booster. May look like the fan relay (I haven't looked closely at an 84 L69). If it doesn't run with the ECM unplugged, wiring an alternate method through the relay should be fairly easy. No need to drop the tank, certainly.

Confuzed1

iTrader: (4)

The in tank pump should still run without the ECM. My L69 didn't even have an electric fuel pump, and it's debatable if you even really need one. GM started putting them in due to fuel boiling in the lines and causing vapor lock as they go near the passenger side exhaust.

Guess they figured if they can send more fuel through the return line of the mechanical pump at low engine rpm's, it would prevent the fuel from getting too hot. The smarter thing to do IMO, would have been to relocate the freaking fuel lines further away from the exhaust!! - duh.

The old CCC systems only controlled the carb, distributor, egr.... emissions stuff...that's all. My electric cooling fan even still worked after I pulled out the ECM harness.

bestracing

I don't see it listed in this drawing as being connected to the ECM..

http://austinthirdgen.org/mkportal/m...ine_wiring.gif

gt4373

According To My Service Manual 84 L69's Didn't Come With An In Tank Fuel Pump.

dmwracing42

iTrader: (3)

Interesting. I had heard that somewhere in mid '84 some of the L69 started getting the in tank pumps. I was also told that if you had 3 wires going to the tank, then there was an in tank. I have 3 lines.

naf

iTrader: (7)

Instead of debating and assuming, why not check for the fuel pump relay and jump the contacts to see if fuel comes out? Sheesh.

dlinger

iTrader: (4)

Our 84 HO Z28 had an electric in tank pump as well as mechanical on block. When researching numbers for the pump at O'Reilly, it was a 4-6 psi pump. It has three wires to the tank.

dmwracing42

iTrader: (3)

With that knowledge, do you have any idea if the ECM would affect it?

dlinger

iTrader: (4)

Sorry, I do not. Old one was not working, pickup tube was not connected. We had one to reinstall, but would have had to modify it to work with sending unit that was available and due to time issue, left it out and are just using mechanical pump.

kmcn47

the ecm shouldn't affect your fuel pump, think of it as a stand alone fuel management system, its one job is getting the gas up to the other pump, the other pumps (mechanical one) has the job of getting it to the carb (in the right amounts of course) and the cc q-jet and your ecm control timing curve and all that other good stuff