4.9L Cadillac poor mileage (Page 1/2)
sanderson231 JUN 12, 05:57 PM
I swapped a 4.9L and PCM from a 92 Deville into an 88 coupe. The fiero gets less than 20 mpg on mostly highway driving and it has since the initial swap. The Deville got about 24 mpg driving under similar conditions.

I checked the O2 sensor with a scan tool. It goes into closed loop after the coolant heats up to about 180 'F. At idle the O2 sensor has a cross count about every 2 seconds. The integrator stayed pretty close to 128. Driving down the road with a few small hills the integrator bounced around about +/- 10. But it seemed like the average was about 128. I'd previously put a new O2 sensor in chasing the poor mileage problem with no effect.

The brakes aren't dragging.

The only thing that seems weird is the MAT reading. With the engine off, it reads ambient. At idle it climbs to 225 'F after the engine warms. Going down the road at 50 mph it read about 185 'F. Admittedly it was 103 'F in the shade today.
Nevertheless, 80 'F above ambient still seems too high. At idle there is no vacuum to the EGR. Furthermore the factory service manual says even with a stuck open EGR there is little EGR flow at idle because the exhaust back pressure is very low.

Anyone got any ideas?
Gall757 JUN 12, 10:33 PM
Did you swap the transmission too?
theogre JUN 13, 01:23 AM

quote
Originally posted by Gall757:
Did you swap the transmission too?

Because that can matter...
And If is an Auto... Did you wire TCC lockup right? Failing that make the Engine to run 200-300RPM More then a locked TCC in top gear(s) and hurts MPG.

Related: Many replace TH125c w/ 4T60 etc and mess up TCC wiring Or just pull TCC plug because bad valve and have same results.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

Dennis LaGrua JUN 13, 03:23 PM
I have received reports where a properly set up and good running 4.9L w 4T60e transmission (stock gearing) gets 29-30 hwy MPG. The Cadillac 4T60e is geared for cruising the highway at a relatively low RPM for comfort and mileage. I believe at around 70 mph the engine is running below 2000 RPM.
Mileage of 20 mpg should only happen in city driving. That isn't great but for a 300 cu in engine its not terrible.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

sanderson231 JUN 13, 06:41 PM
The transmission is the 4T60E from the same car that I pulled the engine. This engine and transmission gave 24 mpg in the Deville which is much heavier and less aerodynamic than a Fiero.

I believe the torque converter lock-up is working. I know I wired it correctly. I'll check to see if it dropping the RPMs when it gets to cruising speed and the gas pedal is backed off.
sanderson231 JUN 14, 05:35 PM
The torque converter lock-up is working.

I have checked the odometer versus GPS
theogre JUN 14, 06:48 PM
Test for TC locking is touch the brake pedal to turn on brake lights on highway. RPM should rise and fall about 200-300 RPM
Just Don't do this w/ people behind you.

PCM needs Correct Speed data more so w/ OBD1 system use by Stock Fiero. Shift points etc change in a light car, different wheels, and more reasons. That's why many PCM can set tire size. Scan PCM and watch Speed matches Speed of car.
Raydar JUN 15, 01:23 PM
The high IAT temps are normal for the 4.9. The air sensor is in the lower intake, right next to the EGR passage.
I don't really have much to offer, but my 4.9 ran an easy 25 MPG with the automatic.

Mention was made regarding tires...
With the Fiero, you are running much smaller tires than you were with the Caddy. You are actually going slower than your speedo indicates, and not as far as your odo indicates. That will tend to make your mileage worse than it was in the caddy.

Do you know how to set timing by ear? You might try that. I had to do that with mine, after I installed the Allante intake.
sanderson231 JUN 15, 08:26 PM
I checked the base timing with a timing light and it was slightly advanced at 11 degrees BTDC vs the book value of 10 degrees. I took the car for a drive to observe commanded timing on the scan tool. Under light load at 60 mph the timing runs in the high 40s and sometimes maxes out at 50. When you get on the pedal it drops into the mid 30s

I can see the RPMs drop and feel the TC locking up.

The PCM, transmission, and engine are all Cadillac. The VSS from the transmission is wired directly to the PCM as it was in the Cadillac. The VS output from the PCM goes through the commonly known resistor/capacitor circuitry to convert from 4000 pulses per miles to sine wave so that it can be read by the Fiero speedometer. The PCM doesn't care what the speedometer reads.

The Fiero has 225/60 R15 tires. The Cadillac had 205/70 R15. The Cadillac tires are about 2.6% larger in diameter than the Fiero. This would account for about 0.5 mpg. When I checked the odometer vs GPS for a 30 mile trip they were within 1 mile or 3%.

Raydar, I would like to know more about the Allante intake swap particularly whether the Fiero throttle cable will work. Also curious about the ignition timing change. I have an Allante intake sitting in the garage waiting to be installed.

All, thanks for the help.
olejoedad JUN 16, 09:30 AM
The ECU does want speedo information to calculate engine load and shift points.
From your description, your setup appears to be wired correctly.

Who tuned the ECU?

Tire diameter is programmable in the 2240 ECU.

Do you have a leaky injector? How long does it take for a fuel pressure gage to bleed down when you turn off the engine?