Help with Carb on computer truck (now wont start at all)

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TacosnBeer

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The maf code is because the pcm is looking for a maf that isn't there. They have a tps somewhere and the pcm hooked up to drive the transmission and the gauges.

skylark - I figured the MAF code was as simple as that. Not sure if that can be turned off or not. It has an electronic-manual TPS that is hooked up in the firewall.

I appreciate everyone's help!

Upper left the throttle flimflamadoodle - oh, and the HOT Air Intake LOL - lame

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It's a Holley FP Reg... no return line.

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Pinger

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skylark - I figured the MAF code was as simple as that. Not sure if that can be turned off or not. It has an electronic-manual TPS that is hooked up in the firewall.

I appreciate everyone's help!

Upper left the throttle flimflamadoodle - oh, and the HOT Air Intake LOL - lame

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You're pulling unmetered (and unfiltered) air through your PCV - (unless it enters the carb above the throttle plate).
Don't know how it should be with a carb but on a 5.7 Vortec the PCV is on the drivers side and goes to the manifold and the passenger side is downstream from the MAF so that the air is counted for adding fuel.
 

Supercharged111

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The PCB setup is as it should be, save for the fact that the breather hole should be filtered. A 3/4" heater hose fitting will snap in there if you want to plumb it to the air filter.

Not sure if I missed this, but what is your base/total timing? Have you tried rejetting? A wideband makes short work of this.
 

gearheadE30

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gearhead30 - yep. stand alone HEI... I will try and pick up e fuel pressure regulator with a return port this week. I have two fuel lines that run up the firewall by the dizzy. One is hooked up, the other isnt... I assume that is the return line. Should be an easy swap. I need to do that anyway. I am also redoing and cleaning the grounds with new straps and wires to rule out grounds as an issue for the gauges as well.

No other pinging or drivability issues. Runs good till you get in it and it falls on its face or its under load.
Not as worried about the dummy lights for now. Prob deal with that later.

I do need to handle the A/C though... I looked for threads with a HOW TO rewire the a/c out of the computer but came up empty. it is still run through the computer so, what would make it not work if the computer is still in there? is there a connection with what the motor is doing to the A/C for ECM management? Is that why it needs to run stand-alone?

Thanks so far... gets me on the right path. I hate computer-controlled cars but... never liked the dash in the earlier trucks. Now I gotta deal with it on a whole different level with this thing.

There should actually be 3 lines coming from the tank on that truck. Pressure, return, and a tank vent that previously would have gone to the charcoal canister. The other line on the firewall is probably the return though. I'm not sure if the fuel pressure regulator will solve this problem, but it will definitely be a durability improvement since the OEM pump is not designed to be nearly dead headed like that and will live a pretty short life.

No pinging or other driveability issues suggests to me that timing is at least close to correct.

Supercharged111 makes a very good point about the wideband, which may be useful to dial in your fueling anyway. Extremely useful tuning tool that I would suggest you add if you can. Not sure if that carb has actually been tuned or if the previous owner threw it on there and expected it to work.

I don't know what signals the ECM needs to 'allow' the AC compressor clutch to engage, but before assuming it is the problem, it would be good to make sure the AC system is actually charged correctly. Nothing works if it isn't correctly charged.

If it is charged, there should be a mechanical low pressure switch and a mechanical overpressure safety switch. I believe overpressure is on the back of the compressor and low pressure is on or near the receiver drier on the passenger side firewall. Both of these need to be hooked up and working. It has been a long time since I've troubleshot them, but I think both of the switches are closed (i.e. have continuity) when they are in range. If they are out of range, then they open and prevent power from reaching the AC clutch.

I assume you have a multimeter so you can check this stuff, but if not, it's an incredibly useful tool that's worth spending a few bucks on.

If you are 100% sure that the rest of the AC system is correct, the LAST thing to check would be briefly jumping 12V to the compressor clutch pin to make sure it is working properly. This bypasses any controls and safeties in the system, so you want to do it only for a second or two. Running it without proper refrigerant charge will kill the compressor quickly, and if it is overcharged or there is a blockage causing extreme overpressure, you could easily blow something up and injure yourself. Hence why there are safety switches.

If you can find a wiring diagram for the AC system, it may make it easier to troubleshoot and confirm that the ECM has control and doesn't just take AC operation as an input.
 

TacosnBeer

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There should actually be 3 lines coming from the tank on that truck. Pressure, return, and a tank vent that previously would have gone to the charcoal canister. The other line on the firewall is probably the return though. I'm not sure if the fuel pressure regulator will solve this problem, but it will definitely be a durability improvement since the OEM pump is not designed to be nearly dead headed like that and will live a pretty short life.

No pinging or other driveability issues suggests to me that timing is at least close to correct.

Supercharged111 makes a very good point about the wideband, which may be useful to dial in your fueling anyway. Extremely useful tuning tool that I would suggest you add if you can. Not sure if that carb has actually been tuned or if the previous owner threw it on there and expected it to work.

I don't know what signals the ECM needs to 'allow' the AC compressor clutch to engage, but before assuming it is the problem, it would be good to make sure the AC system is actually charged correctly. Nothing works if it isn't correctly charged.

If it is charged, there should be a mechanical low pressure switch and a mechanical overpressure safety switch. I believe overpressure is on the back of the compressor and low pressure is on or near the receiver drier on the passenger side firewall. Both of these need to be hooked up and working. It has been a long time since I've troubleshot them, but I think both of the switches are closed (i.e. have continuity) when they are in range. If they are out of range, then they open and prevent power from reaching the AC clutch.

I assume you have a multimeter so you can check this stuff, but if not, it's an incredibly useful tool that's worth spending a few bucks on.

If you are 100% sure that the rest of the AC system is correct, the LAST thing to check would be briefly jumping 12V to the compressor clutch pin to make sure it is working properly. This bypasses any controls and safeties in the system, so you want to do it only for a second or two. Running it without proper refrigerant charge will kill the compressor quickly, and if it is overcharged or there is a blockage causing extreme overpressure, you could easily blow something up and injure yourself. Hence why there are safety switches.

If you can find a wiring diagram for the AC system, it may make it easier to troubleshoot and confirm that the ECM has control and doesn't just take AC operation as an input.

gearhead30 - this is EXTREMELY helpful. I am going to attack the pressure regulator this weekend because I know it isn't right... I will reshoot the timing as well. As I mentioned, it runs fine with no pinging or knocking. Its just the flooring it and when its under load on a steep incline that it misses/cuts out. I will throw a vacuum gauge on the carb as well to dial that in if it isn't... I will report back when I test/adjust all that. Gotta hit napa and get some ground straps to redo a couple of them to rule out grounding problems too.

As for the A/C although important, will be priority 2 to getting it running good. PO said he charged the system and it is ready to go aside from simply not turning on?!? I have a gauge and will check that as well as a multimeter to check the switches. I wouldn't rule out the switch panel itself too.... He seemed to think it had something to do with the computer/engine swap but, didn't seem sure - he simply gave up on it.

Again. I 100% appreciate the help and assistance. The technical knowledge and experienced background information on this forum are amazing. Thank you!
 

TacosnBeer

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Are you getting a solid accelerator pump shot when you pump the throttle?

she is spraying strong when I pull the accelerator - when I first looked at the truck 3 months ago, PO had a 750cfm carb on it. That was too big and he put on the current 650cfm on it thinking that was part of the problem. (Its an Edelbrock Thunder AVS Carb #1826S)

The vacuum line for the HEI is in the left port of the carb which is labeled at the "timed vacuum port" with vacuum introduced off idle/timed. The right port is manifold vacuum which gives vacuum off idle/full. The right port is capped off.

Recommended PSI is 6.5
 
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TacosnBeer

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ok, just got off the phone with my local aftermarket parts joint and asked for a pressure regulator with a return line for a carburated engine... The guy asked for more details and I told him about my Frankenstein setup. He said the stock pump is the problem and should address that since I already have the right Holley regulator. As we already know, the pump is pushing too much PSI for this setup.

The question I have (and he suggested) if I disconnect the power to the stock "in the tank" pump and add an in-line electric 14psi pump = will it suck it through the one currently in there?

OR... is there a in tank pump from an earlier model that has less PSI to accommodate a carb'd engine. I looked up a 1990 GMC 5.0 and the pump in those had a 12psi rating (according to the online specs of the pump that came up). Would this fit in my tank and work with my set up? Seems like it would as long as the tank is the same size and the OHMs for the fuel gauge are the same right? The plugs are different ... 98 had 4 pin, 90 had 3 pin. Dont know of other fitment issues though.

Any thoughts?
 
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Supercharged111

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I used a stock high pressure in tank pump with a Mallory regulator that got me down to 5-6psi on my 92 Crown Victoria. It originally had a 4.6 EFI ******* block engine and I put a carb'd 289 in it. The pump is NOT the problem if the regulator can hang.
 

gearheadE30

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Supercharged111 is right, the factory pump is fine. Yes, it's rated for a certain flow rate at 50 psi or something, but it will not have any problems if you run a regulator and it only ever sees 7 psi. That actually reduces load on the pump, so it will probably live a really long life with low pressure. The pump will just be bypassing the extra fuel back to the tank which doesn't hurt anything.

I don't think swapping the regulator out will fix your problem if it's actually maintaining 7 psi outlet pressure, but it will prevent you from burning the fuel pump up by dead heading it like it's doing now.

It would be much more of a pain to switch to a TBI pump or an inline pump, and if I remember right the TBI pumps have a different sending unit lock ring setup anyway and don't interchange with the tank. I could be wrong though.
 
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