1949 sho

[Off Road SHO]

The secondaries are held open by their springs. As soon as you start the car, the engine's vacuum closes the secondaries. When the rpm'***** 3950, (on a manual car, not an automatic car) the computer sends a signal to the secondary controller. This signal opens up a little valve that dumps the vacuum and the springs open up the secondaries.

Tom

 

[Falconvan]

Thanks, Tom. What differs on the automatic computer to open them?

 

[Off Road SHO]

The auto computer won't let you rev it past 3900 in neutral or park, only the forward and reverse positions. This saves the torque converter and tranny.

Tom

 

[Falconvan]

Yeah, I had Mike at Tweecer bypass all the auto trans functions on the computer. So they should open up at the same rpm as the manual trans otherwise?

 

[Off Road SHO]

You used a computer from an ATX car? Why? Was there an added benefit?

If he deleted or commanded an "ignore" in on the original PCM architecture, you should see the secondary's actuating arms (the arms coming out of the vacuum diaphrams on each bank) snap open the butterflies when the rpm'***** 3,950.


When I Supercharged The Other Woman, I took out the plates of the butterflies. With forced induction, air velocity is not so much of a problem anymore.


Tom

 

[Falconvan]

I just used the one that came with the car. I'll check and see if they are opening; wish I had a blower on mine!

 

[Falconvan]

Looks like the problem is that I'm still getting the rev limiter right at 3900 rpm. Is there any easy way to bypass that? I tried hooking pin 30 wire and pin 46 together with a 220 ohm resistor but I'm still getting. Any ideas?

 

[rubydist]

The atx pcm needs to "see" Drive in order to unlock the neutral rev limiter. If you read the various posts about converting atx to mtx, you will be told what kind of resistance is required on the correct pcm pins to make that happen. I just don't recall off the top of my head.

 

[Falconvan]

Thanks, I think I have it squared away. I called Mike at Tweecer and he told me to send the chip back to him and he'd fix it. When he programmed it he had put the Park and Drive rev limiters up to 8k but left the neutral one at 4k thinking it would not be in the picture. Sounds like an easy fix.

 

[Falconvan]

Got my chip back from Tweecer, looks like that was the issue. Revs fine to 8k now. One more minor issue to fix; a little tough to start when the engine is cold. Once it starts to warm up its fine. I'm going to check the IAC and make sure it's working right.

 

[Off Road SHO]

Also make sure that the temperature sensor the two wire version is plugged in and working properly. It tells the PCM how rich to make the mixture in relation to engine temperature. The single wire sensor is for the dash gauge and the two wire right next to it is for the pcm's use.

Tom

 

[Falconvan]

Also make sure that the temperature sensor the two wire version is plugged in and working properly. It tells the PCM how rich to make the mixture in relation to engine temperature. The single wire sensor is for the dash gauge and the two wire right next to it is for the pcm's use.

Tom

That's the one back by the thermostat housing behind the passenger side head?

 

[rubydist]

make sure the fuel pressure is holding while the engine is off. the most common cause of hard start on the SHO is due to fuel pressure draining down while the engine is off, so it takes a few seconds for the pump to get the pressure back up where it needs to be.

 

[Falconvan]

make sure the fuel pressure is holding while the engine is off. the most common cause of hard start on the SHO is due to fuel pressure draining down while the engine is off, so it takes a few seconds for the pump to get the pressure back up where it needs to be.

Will do; what is the remedy for that?

 

[intimdatr]

That's the one back by the thermostat housing behind the passenger side head?

IIRC, Yes its that one.

 

[Falconvan]

Ok, I'll check that. Here she is all cleaned up and ready to roll; taking it to a couple of local shows this weekend.

 

[Off Road SHO]

That's the one back by the thermostat housing behind the passenger side head?

Yes, on the coolant crossover between the heads, where the thermostat resides.

I misinterpreted. If it is hard to start, like rubydist said, check for proper fuel pressure. I thought it was running poorly till warm. My bad.

 

[Falconvan]

Yes, on the coolant crossover between the heads, where the thermostat resides.

I misinterpreted. If it is hard to start, like rubydist said, check for proper fuel pressure. I thought it was running poorly till warm. My bad.

It'll start but dies right away. After about the 3rd or 4th restart you can rev it a bit and then it will settle into a good idle. Like trying to start an old carbureted motor without a choke.

 

[Off Road SHO]

Something else might not be quite right. But do the easy stuff first: fuel pressure when key turned to ON but not started, fuel pressure when started and as it dies.

Tom

 

[rubydist]

Will do; what is the remedy for that?

It could be due to the checkvalve in the pump leaking, or it could be due to an injector leaking. So the remedy depends on the cause.

Also, be aware that if you have had the battery disconnected and have not run the engine much, the pcm has not "learned" what the proper base pulsewidth is that is needed for the iac. So, if the iac takes quite a bit more pulsewidth than the pcm's guess to make the engine idle, it will not want to run at cold start. The only solution for this is to let the pcm "learn" over time. I had one iac that was off enough that it would take a week for the car to start and idle right each time that the battery was disconnected. I finally tossed the iac and got another one because it was so annoying.