Thread: Chevette blues

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I don't exactly recall the price of a NOS rebuild kit, but it would make for an expensive experiment.

Silly me, I thought ECM stood for electronic computer module. Apparently the "c" stands for control.

Originally Posted by firebird77clone

I don't exactly recall the price of a NOS rebuild kit, but it would make for an expensive experiment.

Silly me, I thought ECM stood for electronic computer module. Apparently the "c" stands for control.

From Rock Auto, looks like about $25 - https://www.rockauto.com/en/catalog/...epair+kit,5964

Are you saying that your stock OEM ECU/ECM has provisions for controlling carburetor mixture? That would seriously surprise me! Do you have a link to something that explains how that feature functions, what inputs it uses, and how it accomplished mixture control back in the '70's with a carb? The only electrical connection I've seen on a carb is for a choke heater.

Ok, much reading and I finally found in the factory manual that indeed the mixture control solenoid functions at 10HZ controlling fuel air mixture.

Also the ECM has a PROM chip, and is part of what the manual calls the computer command control.

it functions largely on closed loop reference to the 02 sensor. At WOT the throttle position sensor which is a rheostat overrides all input and sets run mode to full rich.

oh, also this vette is an 87.

I'm not going to argue with you, but for me your "facts" don't make sense. A solenoid is an on-off device that is either on or off. I'd venture that it cannot modulate to "control fuel air mixture". A PROM chip is simply Programmable Read Only Memory - it's burned with a program, and once burned cannot be changed. Not at all sure what you mean by "...it functions largely on closed loop reference to the 02 sensor", or how it "...sets the run mode to full rich". Your words seem to apply a lot more to an EFI system than a carb, which is a mechanical device using venturis, orifices (jets), and stepped metering rods to transition from idle to mid range to wide open throttle to provide both economy and performance across the operating RPM range.

Good luck sorting out the Chevette. I won't bother you again.

Yes, the solenoid is a binary state actuated device.

In this specific application, operating on a 10 hz square wave signal. Pulse width modulation is utilized to modify fuel mixture from full rich at zero pulse width to lean at 100% pulse width.

On normally operating engine the dwell meter needle , at both idle and part throttle, will be between 10 and 30 ddgree dwell and varying. This is called closed loop and means the oxygen sensor affects control of fuel delivery.

Open loop
The oxygen sensor does not affect fuel control and dwell reading will not vary. This condition is when the engine is cold, the 02 sensor is below 600* F or at WOT.

I mentioned the PROM chip because it was commented that the ECM was not a computer.

Hey, no bother at all.

Getting feedback is why I post questions.

I appreciate your more detailed explanation, but I've gotta say it still doesn't make a lot of sense to me which is OK. Your pulse width modulation sounds like the control mode for a fuel injector where the solenoid pulses open for a measured time, and the high pressure fuel is then admitted to the nozzle to be injected into the cylinder, or into the throttle body plenum. I believe that you'll find that the OEM carb is a Holley 6510, and the "electronic control" is simply the ECM raising & lowering the metering rods in the main jets, instead of them being linkage controlled. Your "pulse width modulation" feature may have to do with the positioning of the metering rods vertically.

I'm not sure about your dwell concept, as in the old days the dwell angle was the amount of time the points stayed closed before snapping open to initiate spark. I suppose your concept is the time, expressed in the angle of rotation, between spark signals. Regardless, the difference between open loop control and closed loop control is feedback. I believe what you're trying to say is that your ECM assigns a dwell setpoint in order to achieve efficient burn of the fuel air charge, and then uses the O2 sensor signal to confirm - the feedback to close the loop and if the O2 is off the dwell setpoint is adjusted to bring the O2 into range. Open loop would be where the ECM assigns a dwell setpoint based on a predicted performance (a curve that says dwell vs rpm should be this line), and assumes it's good regardless what really happens. All good as long as everything's in good shape and adjusted right.

If it were mine, I'd buy a Holley 5210, the same carburetor without any of the electronics, and let the ECM wonder why nothing is listening to it anymore!! Good luck with the Chevette, and I'll leave you alone now.

Thanks for the feedback.

You've got it correct, the solenoid operates a metering rod.
The pulse width sets how long the solenoid is energized during each pulse width(10 Hz). There is no further modulation of the needle, it's either in lean position or rich (0V)


You are further correct on that the dwell metrer is utilized to determine the duty cycle in the same manner as points. Reason being is most shops had an engine analyzer with dwell function. Probably very few shops in 87 had an oscilloscope. Also of note, the manual specifies to set six cylinder mode for correct range readout.

You're also exactly correct on the loop control.

BTW I have a NOS 86 chevette carb on the kitchen table. Same carb, year prior to the electronic gadgetry.

But I might not be needing it.

So check this out.

So today I pulled out my Sears engine analyzer, set dwell on 6 cylinder setting, hooked up to the dwell reading plug, and THE DAM THING RAN.

Wierd.

More interesting than that, it idles slightly better when the dwell meter is hooked up.

Also interesting, is what the dwell meter is showing: it never varied from 10* meaning that it is functioning in open loop (lean) except i thought i had read that open loop was 30* (rich).

More reading needed.

With it running I confirmed that the transmission didn't function for low fluid, now corrected.

So, a quick drive check revealed functioning brakes, anemic acceleration, (to be expected, running at 10* lean) badly balanced tires, no turn signals. However, very nice ride, very straight alignment.

I think next would be to flush radiator, change oil, (zinc additive) and find out why its being held in open loop. Probably bad 02 sensor.

Also, I found that the EGR is receiving full vacuum at all times. I'll have to figure that out before putting it back on.

I found a pulse width modulation chart which shows 10* to be rich, and 30* to be almost middle of rich to lean with 50* and above being designated lean. So the system is definitely open loop rich.

Maybe the anemic acceleration is all it has. I'm accustomed to V8s. It's been many years since I drove this car.

So has all the diagnoses been "just for fun?" Kinda I wanna know what this is supposed to do / how is it meant to be? affair. You've been putting in the research time for sure.

At this point probably 5 hours just reading the manual.

I used to own this car previously. It was my daily driver and extremely reliable. I sold it to my brother in law when I joined the army. He paid me $100 and promised to maintain the lawn while I was deployed. That was 2009.
This spring my brother in law decided to start his own business and needed some cash, so I bought it back for $1K, a good deal considering the condition of the car. However, it hasn't run for at least two years. He wasn't able to figure it out, and just left it parked when the new carb didn't fix it.

While it's tempting to just say "Drop a SBC in it!" at 1987 you're in the years when you're required to install either the OEM or newer, including the EPA equipment or risk big fines or being unable to register it, like the post that Mike shared about the RAM truck guy, and Ryan & Seth regarding shops being hit with huge fines for defeating features on diesels. Maybe it's a candidate for a nice 4 cylinder turbo takeout package!! Make it a Hot Rod!!

it's also tempting to fix it factory correct.

This is the first vintage vehicle I have ever come across which is absolutely 100% intact, every vacuum line is still there, nothing has been removed or bypassed, that is just so rare! And the challenge of learning this obsolete technology is endearing. However I just might draw the line at the ECM.

I've confirmed voltage to the TPS, checked the TPS for function, replaced the 02 sensor, and plugged in the temp sensor (oops) and still running in open loop.

I have a NOS carb base gasket but without the heater grid, I might try that next. The old base gasket really looks like kaka. A vacuum leak can mimmick a lean condition and cause the ECM to hold open loop function.