MAP Adjuster on OBDII

[sudo chop]

Hey guys, I haven't been around in a while, but I'm back to ask a question. I am putting the wbo2 on the Jeep this weekend, with hopes of finding some room to richen it up a bit. Obviously I'm planning on putting a MAP adjuster on it, but I've been reading about it, and it's not clear to me how the OBDII system will respond to the MAP adjustment.

My understanding of EFI from tuning Hondas tells me that the computer ALWAYS relies on the MAP sensor to determine what cell in the fuel map to use. X axis is MAP, Y axis is RPM... I'm not sure what all of the talk is about only using the MAP sensor in open loop mode etc. The readings the computer takes from the O2 in closed loop mode trim the fuel as needed to get stoich, but the computer can't use the O2 as a base to set the pulse width, that just doesn't make sense.

Anyway, it seems to me that the talk about it is from people who don't really get it, but that's not to say that putting on a MAP adjuster doesn't result in some funny business from the computer.

So, all of this to say that I'd really like to hear from some of you guys who run or have at least tried to run the adjustable MAP on an OBDII Jeep, and can tell me what to expect from it.

Thanks

[CobraMarty]

The map adjuster at WOT will not give you more fuel. You need to either increase the fuel pressure or go to bigger injectors so that at WOT your fuel is right and then use the MAP adjuster to take out fuel at mid and low rpm/load.

[SilverXJ]

A MAP adjusted (simple or multiple point) can add fuel at WOT/open loop. Why would you use a MAP adjuster to take fuel out at low load/closed loop?

[sudo chop]

A MAP adjusted (simple or multiple point) can add fuel at WOT/open loop. Why would you use a MAP adjuster to take fuel out at low load/closed loop?

Thanks Silver for the response. I am still confused by this notion that the computer only reads the MAP in open loop. I'm not saying it's wrong, bc what do I know? But I really don't understand this, and I think I do understand how fuel injection basically works, I have tuned a few Hondas, but Hondas aren't Jeeps, so I know things could be working much differently.

I mean, in open loop mode, my understanding is that the computer just runs the fuel map that is stored in memory, with no input from the O2, using the RPMs and the MAP to locate the cell in the map to use. In closed loop mode, it still uses the fuel map, but begins to take O2 readings to try to get to stoich, but nothing about that tells me that it begins to just ignore MAP, in fact it seems absurd to me to imagine that it could possibly stop using the MAP input, because that means it would stop using the fuel map, and not using the fuel map to base pulse width off of seems simply ridiculous, it's like saying that it stops caring what the engine speed is.

Again, maybe some of these principles I know from only Hondas don't convey to the Chrysler fuel injection system, I'm really just trying to understand.

[SilverXJ]

The computer doesn't entirely ignore the MAP while in closed loop. However it relies heavily on the O2 sensors for the correct afr. The computer looks at the O2 sensors and modifies the fuel look up table via long term fuel trim. However, if the MAP is adjusted in closed loop the close loop adjustment will just be taken out in the fuel trims. Also, the MAP contributes to timing control as well.

[CobraMarty]

No amount of adjustment of the map sensor will give more/enough fuel at wot. you need to use bigger injectors and then use an adj map to trim out some fuel at mid and low range. This is simple EFI info. There are plenty of books and articles out there.

[SilverXJ]

I don't know where you get that info, but it is simply not true. I ran an Apexi (MAP adjuster with multiple points) on my storker and was able to add fuel just fine. No book I have read gives your opinion in it.

[sudo chop]

The computer doesn't entirely ignore the MAP while in closed loop. However it relies heavily on the O2 sensors for the correct afr. The computer looks at the O2 sensors and modifies the fuel look up table via long term fuel trim. However, if the MAP is adjusted in closed loop the close loop adjustment will just be taken out in the fuel trims. Also, the MAP contributes to timing control as well.

Ahhh, I see. So the idea is that the computer will eventually just trim out the added fuel. See, but doesn't it store that data to the map? Wouldn't it continue to use that adjusted fuel map in open loop? Or will it always use some separate base map in open loop? You could just continue to adjust the MAP voltage until the computer runs out of room to trim, right?

I've just ordered the stuff from digi-key, so I guess I will find out soon, for myself. I'll definitely be back with an update with results.

Silver what are you running on your Jeep? SAFC?

[SilverXJ]

Ahhh, I see. So the idea is that the computer will eventually just trim out the added fuel.

It will very quickly in closed loop.

See, but doesn't it store that data to the map? Wouldn't it continue to use that adjusted fuel map in open loop?

It does store it in long term fuel trim. I don't know how it uses that to correlate to WOT open loop though as I never had it dial it out.

Silver what are you running on your Jeep? SAFC?

I was running the Apexi SAFC. I am running the AEM FIC now.

[sudo chop]

I'd like to have a piggyback, but I can't justify that price unless it's really necessary. If this simple MAP adjuster doesn't solve my problems, though, I'll probably be there.

Thanks for your help, Silver. How is the F/IC?

[SilverXJ]

I like it a lot. It has a lot more features than the Apexi. You can reduce timing, add or remove fuel w/o modifying the MAP signal, attach a wide band to it for logging, you still have the option of modifying the MAP signal, it has two additional channels that you can modify or attach for data logging, it has a switched 12v out that you can set when it comes on, you can have two calibration files that you can switch between with a switch, and it has excellent data logging. The only down sides are its O2 sensor modification channel doesn't work with narrow band sensors and you should really mount it inside as it isn't weather proof which means extending the harness. I do miss the gauge display on the SAFC II though.

[Cheromaniac]

No amount of adjustment of the map sensor will give more/enough fuel at wot.

Definitely not true. I've been using my homebrew MAP adjuster for many years to increase the fuel flow from the Ford 24lb injectors on my stroker. At stock MAP voltage it runs lean but at 5.3v it's perfect.

I am still confused by this notion that the computer only reads the MAP in open loop.

The MAP (manifold absolute pressure) sensor is the principal load sensor so the computer reads the output voltage from that sensor ALL the time. However in closed loop computer operation, O2 sensor feedback control ensures that a stoichiometric AFR of 14.7:1 is maintained even if you change the MAP voltage (as long as you don't go too extreme with either MAP voltage, fuel pressure, or injector size).

[Comanche91]

Digging up an old thread here. The OP asked about using a MAP Adjuster on an OBDII system. I didn't see it answered.

I have a similar question. I have a 62mm bored throttle body on my OBD1 stroker. The throttle body vacuum port is capped off and am using the original MAP on the firewall. I would like to get rid of the long vacuum line from the firewall MAP to the intake manifold and use a 96 and up OBD2 MAP sensor mounted on the new throttle body. Would the OBD2 MAP sensor operating on a 5VDC reference put out the same voltages throughout the vacuum range as the original OBD1 MAP? Or are the ODB1 and OBD2 MAP sensors not compatible? Thanks all.

[Cheromaniac]

Digging up an old thread here. The OP asked about using a MAP Adjuster on an OBDII system. I didn't see it answered.

The answer is that a MAP adjuster will behave similarly on both OBD 1 & OBD 2. However in '00+ models with a DISS, I wouldn't recommend using a MAP adjuster 'cause the PCM is locked into a stoich AFR at WOT up to 2300rpm and becomes richer thereafter so you don't have a relatively "flat line" AFR across the rpm range to adjust.

I have a similar question. I have a 62mm bored throttle body on my OBD1 stroker. The throttle body vacuum port is capped off and am using the original MAP on the firewall. I would like to get rid of the long vacuum line from the firewall MAP to the intake manifold and use a 96 and up OBD2 MAP sensor mounted on the new throttle body. Would the OBD2 MAP sensor operating on a 5VDC reference put out the same voltages throughout the vacuum range as the original OBD1 MAP? Or are the ODB1 and OBD2 MAP sensors not compatible? Thanks all.

OBD 1 & OBD 2 MAP sensors behave in the same way and have the same 5v reference voltage, so you could use the OBD 2 MAP sensor on the TB. However you'd need to extend the wiring from near the firewall to reach the OBD 2 sensor and I'm not sure if the connector is a direct fit. FWIW you might as well leave your existing set up as it is.

[Comanche91]

Thanks Dino, I figured as much. I knew they used the same reference voltage but was not sure the voltage outputs to the ECU were proportioned the same over the vacuum range. I'll just make up an extension cable w/o cutting any connectors and see how it does. The connector is the same on the 96 MAP (round pins) but changed in 97 (flat pins).