My Car has a 383, mild cam, stock tpi, and true duals. (no egr or cat). The idle was at 158 BLM with the 24# injectors @ 50# fuel pressure, 2-1-2 exhaust. With old exhaust rusting out, I installed CC true duals 2.25 " and magna flow mufflers. The blm's went off the chart. It will even drowned out the car at 70 mph, when I switched the prominator to that one bin, the injectors went from 3 ms to 12 ms the car lost speed, the engine almost died. When I flipped it back all was fine. The O2 Mv never changed (stayed below 450 Mv). The difference in bins was intergrate and blm were limited to 130 - closed loop, limited richness.

I am now running a heated O2 but the Mv drop to "zero" at idle or just off idle.

Spark Advance (more is worse), O2 position (down the pipe worse), and rpm (below 1000 worse) have the most influence.

I have a WB just not hooked up yet, I wanted to do some more stock base lines before jumping into that world. Had an exhaust leak (cracked header) and been playing with O2 swings, O2 gain , Spark Adv, and injector constant. To see what does what.

I have tried 4 different O2 sensors.

The 89 $6E Arap bin table (Injector Offset for Small Pulse Width's vs Base Pulse Width) is at location 036C
ms---- offset = ms
3.660 + 0.000
3.410 + 0.000
3.170 + 0.000
2.929 + 0.000
2.685 + 0.000
2.440 + 0.000
2.197 + 0.015 = 2.985
1.950 + 0.031 = 1.981
1.708 + 0.046 = 1.754
1.460 + 0.076 = 1.536
1.220 + 0.076 = 1.296
0.979 + 0.092 = 1.071
0.732 + 0.138 = 0.870
0.488 + 0.275 = 0.763

There is nothing linear about these numbers.

 

Sorry it's been so long to update. Too many irons in the fire. I finally got around to manually checking the milage and the display is accurate within 2-3 tenths. With a 50 50 mix of city and hiway driving, I averaged a little over 17mpg. In my spare time (yeah right), I changed the oil pressure switch. It started leaking right after I changed the injectors. What didn't get burned off on the exhaust made it's way to the ground leaving puddles. Next item to replace is the O2 sensor and reset the ECM again. If that doesn't help with the milage, I guess I'll need to find someone that can datalog the BLM and find out what's going on there.

 

Thinking of putting new injectors in my 1985 / L-98 this winter. Should I be scared ?

 

Joe,

After replacing the Multecs in my '90 coupe with Bosch type III's, I like many others am also seeing a marked MPG decrease. From a seat-of-the-pants perspective, the engine seems to run much better; however, the decrease in mileage is a little disconcerting. I’ve been trying to get a handle on why this happening. I am intrigued by your comments regarding the pulse rate of the stock injectors vs. the Bosch III’s. I have been looking around the net for this kind of technical information for some time to no avail. If it is publicly available, would you mind letting us know where you found it?

I use TunerPro RT for datalogging. At a steady 58 MPH, the BLM’s hover around 145 which, as I understand it, seem to indicate that the ECM is asking the injectors to add additional fuel to compensate for a lean condition. Maybe this is where the extra fuel is going. I have been advised to change the O2 sensor on the theory that it may be a bit sluggish and not providing accurate feedback. Any thoughts, tips, or suggestions you may have will be appreciated.

Thanks.

 

maybe Jon can chime in and solve this mistery

 

Russ,

Maybe this from the FSM will help:

Run Mode (6E3-C2-2)

The run mode has two conditions called open loop and closed loop.

When the engine is first started and rpm is above 400 rpm, the system goes into open loop operation. The ECM ignores the signal from the O2 sensor and calculates the air/fuel ratio based on inputs from the Coolant Temperature Sensor (CTS) and Manifold Absolute Pressure (MAP) sensor.

The system stays in open loop until the following conditions are met:
1. The O2 sensor has varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature.)
2. The CTS is above a specified temperature about 40c (104f).
3. The specified amount of time has elapsed after starting the engine (approximately two minutes).

In closed loop, the ECM calculates the air/fuel ratio (injector on-time) based on the signal from various sensors, but mainly the O2 sensor.

Speed Density System (6E3-C1-3)

The Corvette’s Port Fuel Injection (PFI) system is a speed and air density system. The system is based on speed density fuel management.

Three specific data sensors provide the ECM with the basic information for the fuel management portion of its operation.

The engine speed signal comes from the distributor’s HEI module. The ECM uses this signal to determine the speed or rpm factor for fuel and ignition management.

Two sensors contribute to the density factor: the Manifold Air Temperature (MAT) and the Manifold Absolute Pressure (MAP) sensors.

The MAT sensor measures the temperature of the intake manifold air; the MAP sensor measures the pressure of the intake manifold air.

As intake manifold pressure increases, additional fuel is requires and the ECM increases the injector pulse width. As intake manifold pressure decreases, the ECM decreases the injector pulse width.

Oxygen Sensor (6E3-A-26)

The O2 sensor is like an open circuit and produces no voltage when it is below 360c (600f). An open sensor circuit or cold sensor causes open loop operation.

Code 13 (open O2 circuit) will be set if:
• Engine at normal operating temperature (above 70c (158f).
• At least 2 minutes engine time after start
• O2 signal voltage steady between .35 and .55 volts.
• Throttle position sensor above 5% (about .3 volt above closed throttle voltage).
• All conditions must be met for about 60 seconds.
If the conditions for a Code 13 exist, the system will not go into closed loop.

 

this is very common, not just with the bosch 3's, but newer style injectors. I would like an explanation too. The people who say their MPG dropped need to read their BLM's .............

I bet they are high as well.

Has anyone contacted PCM;s for less ??? and see what they say?

I'd hate to burn a prom because I swapped injectors...

 

this issue has been going on for over 10 years. But mileage was never more important than the performance, now they both are equal. I have many customers that report better mileage and some reporting worse. I wonder how many of those multecs were not really working properly. A high BLM count, will make the ECU give more fuel as it is detecting a lean situation. But, is it really lean?.. Is the o2 sensor good?. A few other guys with high BLM that we just worked with both had a running fuel pressure in the 30's. They fixed it and now have good mileage and normal BLM counts. So maybe get a reading on actual FP while driving. See if its low. One thing everyone seems to see is better performance.

 

I noticed a little drop in MPG with the bosch III's but the car starts all the time and runs way better than it did before. Guess I will trade a little MPG loss for something that is reliable.

Phil

 

Russ,

For a small investment in money time and effort, you can do this yourself. I too have a 1990 Coupe. Here is how I did it: http://wagtail.home.att.net/extras/newbie.pdf.

Being able to log your engine's ECM data is way cool!

Trust me.

 

Been having a similar problem with my '91 L98 since I replaced faulty injectors... 20 - 30% mileage drop. Reset ECM, drove several times but no mileage improvement. I hate to drive the car now, even with gas prices declining. Any ideas?

To check for open/closed loop, install the diagnostic jumper in the ALDL connector pins A to B like you were going to read error codes but start the car instead. The CEL will flash at 2HZ rate when in open loop then at 1Hz rate when in closed loop. Raise idle to 2500 RPM. Length of the flash describes rich/lean operation... on-period is lean and off-period is rich, so expect a nice, steady on, off, on, off at a 1Hz rate if mixture is correct. On, on, off, on, on, off indicates running lean and On, off, off, on, off, off indicates rich.

Article is at:
http://www.geocities.com/jgkov/FSMode.html?20085

 

Bringing this thread back up after my Bosch III install I too have lost ~25% of my gas milage.After reading my question is if the ECM is calculating with an injector that operates the same flow at a different pulse rate and showing a calculated lower MPG on the dash should the actual(real world) MPG be different? After all it is using the O2 and correcting the A/F ratio so the engine should be burning the same amount of gas as before overall right?

How about the BLM's do they have a limit?

And just exactly how do the Bosch III injectors pulse/flow rate differ from the Multecs?

I had FMS blue tops in my last car and I know they flow differently but my milage did not suffer as much as these injectors have caused.

Obviously time to break out datamaster and see what is going on.

 

 

.... Still refining my thoughts

I ASSUME the ECM is calculating the mpg based on a fixed value for Multech injectors pulses. If the Bosch needs to pulse MORE for the same A/F ratio it would reflect a rich value in the BLMs right? The only thing that shows the true nature of fuel being used is the O2 and based on that if the Bosch flows less per pulse the ECM would try and richen the BLM to get the correct A/F ratio at the O2. If this is true then the MPG calculation is useless after the install but the actual MPG should remain somewhat the same if manually calculated correct? So the only thing to be concerned about is if the O2 is averaging ~800(right value?). It would also make the reported BLM sort of useless in a scan too correct?

 

are you are using the on board computer to detemine mileage? I don't know if 25% is the actual number. Please drive it a few hundred miles and do a manual mileage test. This injector is the same flow as the multec and smaller than the svo 24's theres no way a 24lb injector that flows 25.5 at 43.5 psi can use the same fuel as a 22. put a datamaster on and lets see what the BLM is. Also give me the part number on your old injector..I would like to see what was in there.. Did anyone play with the tune from when you had 24'S?

 

This is one of the best ways to be sure. Waiting for the fab to cylcle twice is a pretty much sure fire way to confirm.

 

Nope all stock the 24's were on my last car not this one and I had a mild tune done for them.

BLM's are ~150 on the scan I did tonight
INT is ~128 as it should be.

I hear ya on the driving thing I saw my Multecs change after I sealed the intake over time from 23-24 to 27-28. As I PMed you just a little concerned about the BLMs. Mabey do another ECM reset and drive it instead of letting it sit for 15min?

 

This is a VERY interesting thread. FIC has a good point about everything else in the cars being sound when new injectors are installed as a new good component can REALLY make other poor components stick out.
More than that though I am interested to see someones capture(datalog) after these are installed. I will be picking mine up in about a month even if I lose 5 mpg. It seems that my car, most of the C4's I have driven and many of yours all get better mileage than they were rated for(29-30mpg). These injectors seem more that they are bringing that back down to factory reference and returning performance in exchange. I'm all ears

How many here change their fuel filters at the same time they did injectors?

 

When I did my Bosch lll's i also did new filter's air & fuel, plugs, wires, cap, & rotor I also cleaned TB & IAC. I have had them in car now about 6 months or so, my MPG is maybe off a little was about 23.5 @ 70MPH now about 22 it's a 87 L98 I can live with that, more important it's running good. I also at about the same time switched out exhaust less back preasure might have a affect switched out old 02 sensor for a new Bosch one, car idles smooth @ 500 RPM in gear. Over all I am happy with injectors.

 

After doing some at idle datalogs I have some data to share.
I did a propane leak test none detected

MAP@idle675rpm ~39kPa (within the 20-48 range in the FSM)
BLMs ~150 at idle

MAP@2500rpm ~31kPa
BLMs ~158

Did a FP leakdown test
46psi @ key on
40psi while running at idle
Holds after shutdown for at least 30 min

Removed vacuum(plugged) from fpr car ran at 48psi

Everything seems to be good with the exception of the BLMs calling for more fuel. Looking at the O2s the do seem to bounce above 800 a bit and hovers around 750 during the 2500rpm section so yes it is calling for the ECM to add fuel.

I am considering doing a run with the FPR vacuum off to increase the pressure to 48psi and seeing what will happens to the BLMs.

Stay tuned......