3.4 sfi ecm question

I'm guessing Brian is the one who might be able to answer this, but if anyone else has input by all means chime in. When I put together my 3.4 sfi sprite I used an ecm from a 95 grand am 3.1 and have had no problems to date. However I began to wonder if this ecm was able to give me the full capability of the sfi system since I did not know much about the 3.1. So I decided to, as an experiment, get an ecm that was specifically for this motor. I picked up one from a 95 camaro 3.4 sfi 5 spd and installed it. Weird results: Once the fuel pump prime is complete,it starts and runs sweet for about 1 second and dies. Let the fuel pump prime again and it will again start and run for about 1 second. I thought it might not be getting fuel but when I put a fuel pressure gauge on it, there is pressure when it dies. I did not use the vats bypass when I put the car together but rather run the pump off a switch. Thinking the new ecm needed the vats bypass for some reason,I installed it but the action is the same-primed it will start and run for just 1 second. Any ideas? I'm confused-john.

Hi John,

Only what I have read and understood. Yes, I have used a number of the FWD PCMs with good results too.

I understand that the Camaro PCM shuts down the fuel later but the spark pretty quickly so without a VATS signal, you cannot run the Camaro PCM. Most others as well. You will hear the pump kick in but it usually not get above cranking speed- or about 1 second.

You could try our VATS disable which should clear this up: http://bmcautos.com/shop/index.php?main_page=advanced_search_result&search_in_description=1&keyword=vats

Brian, when I put the 3.1 ecm in initially I did not use a vats bypass even though I have one(I think it is from baker electronix-don't know if it is differnet from what you offer) since I found out that the system worked just fine by having the fuel pump operated manually by toggle instead of controlled by the ecm. Do you know offhand if that approach will work with the 95 camaro 3.4 sfi ecm?

Brian,I got it working! Here's what I found out. The 3.1 ecm did not require the vats bypass and would work if I bypassed the ecm with the fuel pump control. The 3.4 sfi would not work without the bypass even with the fuel pump control done manually. All I can figure is that the 3.1 ecm did not have a vats system from the factory-just a guess on my part as I don't have a service manual that covers the 3.1. Fortunately I had a vats bypass laying around in the shop so could resolve this quickly. Thanks for pointing me in the direction of the vats.Once again Brian, you are a great asset to this board,especially to those of us into the 3.4 sfi - thanks. john

Sounds great. Glad you got it working. If you can keep a log and post any differences between the PCMs that you note, it would be very interesting to hear your results.

-BMC.

turbostar Wrote:

Scott,thanks for that info as it clears up the confusion I had regarding the two ecm's. Brian, even though I have driven a few miles with the 3.4 ecm I can tell you there are differences. The differences are not huge but in my mind significant. I can't say that I can tell any difference under hard acceleration or higher rpm's. On the other hand at idle and in the lower rpm range(below 2000) I can notice smoother operation and from what I know about sfi that would make sense. In my opinion it is worth the effort to put the sfi ecm in with the sfi engine(ecm only cost $35 plus shipping). I guess I am still a little amazed at how well the 3.4 operated with the 3.1 ecm,after all that is what was in the car when I got 37 mpg city and 42 mpg highway. If I notice any other differences I will post and when I get time I will redo the milage check with the new ecm. john

John,

Did you have to alter the 3.4 harness to operate with the 31. ecm and then again when you switched to the 3.4 PCM? I seem to recall that the 3.4 fires its injectors sequentially below a certain engine speed then switched over to batch injection at higher rpms, and earlier ecms like the 3.1 you were using always just run in batch mode.

I don't know if this is applicable but on my earlier Camaro unit from an 87 the enable for the fuel pump required oil pressure for the pump to continue past the intial 2 second prime. The prime sequence supplies enough fuel pressure to start the car, then it checks for oil pressure to continue the pump. It's a safety measure so that the pump will shut off in case of an accident where the engine dies.

Brian,I redid the 95 harness as if I had the 3.4 ecm from the beginning. However,when I first got the project running I used the 3.1 ecm and it is amazing how well it ran considering it was not the correct one. My recent switch to the 3.4 ecm was for my concern based on getting the full benifit from the sfi. My thoughts were born out but the difference is minor-but worthwhile. john

I too have run my 3.4 with a 3.1 ECM - in fact that's what I'm planning to continue doing. It does work well, except in Open Loop, when it runs lean (the ECM thinks it's a 3.1, so it outs in 10% LESS fuel than the 3.4 needs). Sad not to use full power (which is one of the Open Loop modes). I will either put in 10% bigger injectors and keep this ECM, or learn to reprogram before too long!

In all areas of running where the ECM looks at the O2 sensor output, it can compensate for the 10% difference in displacement.

Now, I've run the same car with the same part# ECM, but with the program from a 2.8. Wow! That made a a noticeable difference, being 20% out on the Open Loop fueling! I understand that the computer's ability to compensate, even in closed loop operation, can be a little iffy when changing driving styles/conditions (eg coasting to power, etc).

I too noticed incredible mileage with the 3.1 ECM. I was getting 30MPG at 70mph on the highway in a 1984 Jeep Cherokee 4x4, according to the miles on the odometer and the gallons from the pump. Wow!

Duncan,

I'm really surprised that you are running lean in open loop. Are you sure that you are truly lean?

I'm running a 3.4 computer. At the stock fuel pressure setting, the car runs way too rich during open-loop operation. This is a known issue with the 3.4 Camaro and multiple sources have highlighted the same issue.

I use an Innovate Motorsports LM-1 wideband air-fuel meter to verify the true AFR. At stock fuel pressure, the car runs at 10.5:1 when in open loop. My engine is even modified slightly and likely flows a little better than stock. I used a Comp Cam 252 can, bored the motor, and lightly skimmed and ported the heads.

I installed a Holley adjustable fuel pressure regulator and lowered the fuel pressure to 37 psi.

http://store.summitracing.com/partdetail.asp?part=hly-512-501


This improved the situation, but it's still too rich (12:1) during open loop. I haven't had a chance yet to test it at even lower fuel pressure. Even at these lower pressures, the car still runs perfectly during closed-loop operation.

I'm not positive, but I wouldn't expect that the the injectors are a limiting factor,unless your engine is heavily modified - even with a lower ECM flow map. Then again, I could be wrong.

Have you verified that your fuel pressure is in spec?

If you are lean, then I would hypothesize that your fuel injectors may need to be cleaned. Perhaps they are not balanced, so some cylinders go lean. I recently sent mine out to be cleaned and I'm amazed at how much it smoothed out the idle. The test data from the cleaning and test process shows that the cleaned injectors now flow within 1.8% of each other. Here's where I sent them:

http://www.witchhunter.com/

I have to admit I'm not totally sure it goes lean, but it definitely has less power at full throttle than at about 75%. As far as I understand, the Fuel Enrichment mode kicks in about 85% throttle.

The O2 sensor is only accurate over a very narrow range of Air Fuel Ratios (AFRs), but a car benefits from a wide range of AFRs over the range of RPMs and loads. Carbs compensate by opening jets/powervalves/etc, but most EFI cars simply ignore the O2 sensor at full throttle and go to a look up table.

I haven't modified the look up table in my 3.1l Pontiac 6000 ECM, so at full throttle, it dumps in enough fuel to make a 3.1l go rich. Unfortunately, I'm running a 3.4l (which the ECM doesn't know). I'm pulling maybe 10% more air than the look up tables account for, so I'd need 10% more fuel at full throttle for the same AFR as stock for the 3.1l was. Still, I must admit I'm guessing it's running lean when it gets a little less powerful at full throttle.

You're right, it could be something else! There is always more than one possible solution... I haven't checked my fuel pressure from the Jeep Cherokee fuel pump, I'm running the stock Fiero regulator/injectors/intake, and the engine is bone stock (except for an oil stripper and timing chain tensioner from a twin cam 3.4.). I'm thinking injectors from a 350, which are about 20% bigger, would be a simple solution. The look up tables should be a lot closer, and closed loop wouldn't really change. Like yours, it runs great in open loop!

Strangely enough, I had the same "less power at full throttle" problem with my MGB engine! I swapped carbs, manifolds, distributors, coils, wires, and timing chains with no effect. I mic'ed the cam lobes, changed fuel pumps several times, and went to electronic ignition (3 types). For the 19 years I drove it, I never found the answer. It's ironic that I'm saddled with the same problem now!