quote Originally posted by phonedawgz: What does the scanner report as the fuel pump voltage. Does that very? Try running the engine with a battery charger connected at the same time and plugged in. That way there is no way what your chasing is a changing voltage perhaps caused by the alternator and picked up by the ECM and trying to be adjusted for by the ECM. The ECM is supposed to adjust the pulse width when the fuel pump voltage is lower. The ECM however doesn't have a wire that connects to the fuel pump to sense voltage. It only senses battery voltage.

The ECM does Not report "Fuel Pump voltage." Yes... Fuel pump is on a different circuit and pump voltage can be low, even dead, for that the ECM know. (Of course Dead circuit = dead engine...) And note... If you think the Fuel Pump is getting low volt then test it further ASAP. Low Volt, from bad relay/wiring or ground, means a dead pump in future. See cave, electric motors in general electrical.

The ECM report Its own voltage but Scan tool says Battery voltage. If voltage is low then ECM will bump to low voltage condition whether alt, battery, etc, is fine otherwise. Easy to trick ECM has low volt... bad ground and/or loose + wire. Scan ECM voltage and should match voltage at battery -/+ 1 volt. -/+ 0.5 volt is better.

ECM also report High and Low voltage. Low flag sets, i think, under 12v. If low volt is set then ECM will increase Injector time, idle RPM, and ignition Dwell time.

The ECM and engine is not picky about Idle Stop. Set to 700-725 RPM with screw and ECM won't care much if at all. What ECM get pissed if screw RPM is too high and ECM can't make IAC to close enough and sets DTC 35.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

quote Originally posted by phonedawgz:

Can I use that chart on my '88 TBI for port numbers?

I have port A going to my MAP sensor, not port E as shown in the picture.

[This message has been edited by sportcoupe (edited 12-25-2010).]

If the vacuum port to the MAP isn't a full vacuum port it will cause issues.

The 88 service manual is downloadable here

http://www.fieronews.net/fu...nloads.php?cat_id=15

Ogre - I agree but have been told by others on here that their scanner reports it as 'fuel pump' voltage.

That image from 300 tbi.... Is not right. Ooop.

Use trunk lid VECI label, download SM, or see cave in 700 TBI.

phonedawgz, I want to know which kind of scanner... None of GM OBD1 will report fuel pump voltage. Can make a case for battery voltage and label but fuel pump voltage? Wonder if it a commercial one or free/shareware one. It's not the old AutoXray GM scanner... That one is correct. (I own that one.)

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

idk but it reported battery voltage as fuel pump voltage. I got into a big discussion of how the scanner couldn't report fuel pump voltage because there was no wire from the switched side of the relay to the ECM. The guy was trying to say if you had a bad relay, the ECM would sense the drop in fuel pump voltage and adjust the pulse length. One of his 'proofs' was his scanner had a reported 'fuel pump voltage'. It's somewhere here on PFF

Interesting thing however is the 7730 ECM does have an input that runs from the pump supply line to the ECM. So if you are running a 7730 ECM it does know fuel pump voltage.

[This message has been edited by phonedawgz (edited 12-25-2010).]

quote Originally posted by phonedawgz: If the vacuum port to the MAP isn't a full vacuum port it will cause issues. The 88 service manual is downloadable here http://www.fieronews.net/fu...nloads.php?cat_id=15 Ogre - I agree but have been told by others on here that their scanner reports it as 'fuel pump' voltage.

I have the '88 FSM but it doesn't show the TBI vacuum port identifiers like the '84-86 diagram you posted does. The trunk lid diagram also does not show the vacuum port labels as well as the '84-86 diagram posted here.

My scan tool shows battery voltage, 14.4 volts while idling. Factory ECM, not 1227730.

So close the IAC fully, (extend with A-B) and get the car to idle as slow as you can but keep it running. See if you have full time vacuum on the MAP port. The MAP needs to be connected to the intake manifold via a full time non-ported vacuum connection. The MAP needs to sense the actual pressure inside the intake manifold so the ECM can open the injectors for the correct amount of time to inject the correct amount of fuel.

Just physically match port on TBI to hose like VECI says.... You don't need A, B, etc. Just ignore image above.

You have 1227748 ECM. See cave, many pages.

phonedawgz...
I see... 1227730... It's a P4 ECM...
Yes... 7730 does has an extra input line to test fuel pump volt. I'm guessing scanner has Two line also... battery and fuel pump voltages. (If you read schematics, go here: http://web.archive.org/web/....cruzers.com/~ludis/ look at Passive Inputs page, label pumpvolt and volt.) So I guess I'm wrong... Some ODB1 do have measure voltage other than battery voltage.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

The whole fuel pump voltage input thingy is merely a monitor. It will trip secondary fuel pump failure if voltage is below threshold. Having a properly set minimum idle spec is to prevent stalling under high idle based loads such as prolonged deceleration in gear or when placing sudden loads on at low speeds such as AC clutch and power steering loads as well as coolant fan on which causes sudden alternator loads.

quote secondary fuel pump failure

Which pump is the secondary fuel pump?

quote Originally posted by phonedawgz: Which pump is the secondary fuel pump?

I am guessing he is referring to the two ways the fuel pump can get power, either by way of the relay controlled by the ECM or directly off the battery controlled by the oil pressure switch.

But the ECM has no way of telling if the fuel pump is getting power. There IS a lead on the 7730 ECM that can sense this. There is not on the Fiero ECM.

Also it wouldn't matter if the ECM/Relay was supplying power or the Oil Pressure Switch is supplying power. The voltage to the fuel pump and also the voltage that gets monitored on the 7730 ECM doesn't care where it comes from, as long as the voltage is sufficient. So a low volt wouldn't indicate a failure of one of the power systems. A no-volt however would during the 'prime' function.

In detail, secondary fuel pump failure refers to the fuel pump relay monitor. I'm not certain where it fits in to the ECM logic, at this point all I am aware of is that it is a function monitor.

I believe we've drifted way off with all this fuel pump voltage talk.

Christmas is over and I got back to the Fiero this morning. Last thing I did was crank up the idle stop (past 'normal') to get the IAC counts down from 200+ to a reasonable 25. The engine ran good, or so I thought, because it was fully warmed up after a day or tinkering.

This morning I start the Fiero and instead of being greeted with 2000 rpm+ at startup I get a stall. I crank again, it barely starts and stalls again. I crank a 3rd time and give it throttle to keep it running. It runs but isn't happy and wants to stall. RPM's are low, very low. I keep my foot on the throttle and warm it up for a minute. It will idle now but just barely (800-900 rpm). I drive around the block, it runs good at part throttle but idles very low and almost stalls at low idle.

I get back to my house and put the scan tool on it. IAC counts are pegged at 255. Idle stop is almost maxed out too.

Last thing I did before it started really acting up was clean the lower throttle body. The TBI isn't that complicated and I'm at a loss. I have pulled the lower throttle body (again) and it is soaking in a gallon of cleaner as I type this......

What could drive IAC to fully retracted position at idle? I have no codes at all.

Most likely mis-adjusting the throttle stop. I suggest you follow the factory procedure.

Remember the IAC is a stepper motor. The ECM will move the motor in steps. When the IAC's pintle get's to the end of it's travel, the ECM is still sending steps however the IAC pintle is no longer moving. So the IAC counts get off. Now when the ECM tries to adjust the IAC it might again run into the other stop, or at least be way off of where the ECM remembers it left it last.

[This message has been edited by phonedawgz (edited 12-27-2010).]

I checked for that when I swapped IAC for troubleshooting. When the scan tool reports high IAC count I pulled the IAC and the pintle was fully retracted. I re-installed and jumpered ALDL, pulled IAC and pintle was fully extended.

So if the idle is really slow with the pintle fully retracted, that means air isn't flowing through your IAC port. Take out the IAC and start the engine. If it runs very fast and is sucking air into the open IAC hole, the blockage has to be before the IAC. If no sucking from the IAC hole, well then it has to be past it. You can regulate the speed of the engine by sticking your finger in there.

I readjusted idle stop per the book and upon re-start the idle still sucks. A scan reveals IAC is maxed out at 255 counts. I pulled IAC and it is indeed fully retracted.

The IAC passage is very huge, I don't see how it can be blocked. I agree with your theory though 100%. I am going to pull the IAC and start the engine without right now.

I didn't replace any throttle body gaskets as they were still good so an incorrect gasket can't be the problem. Good idea though.

Phonedawgz is a genius.

I removed the IAC and started the engine briefly, rpm's shot up high. Blockage must be before the IAC. I look at the IAC port in the upper TB and all I see is gasket. Duh.

I pulled the upper TB off and the gasket that separates the upper from lower is rotated (not flipped) 180°. That gasket can physically go either way but there is only one IAC passage in the gasket so it MUST be positioned correctly.

I correct the gasket, re-assemble, re-adjust idle stop per the book, drive it around the block to learn idle. Connect my scan tool and IAC counts are 2. perfect. I still cannot set idle stop below 850 rpm without engine rpm surging but everything else is perfect (again).

Genius? I really don't think so. But thanks for the kind words.

Maybe more like 'experenced like an old whore' is a better way to put it. Yeah I can tell you what those nasty spots are on your wrench

I wonder if that was the only problem of the guy who saw that before. Maybe he put a new hole in his gasket when all he needed to do is flip it.

[This message has been edited by phonedawgz (edited 12-27-2010).]