4.9L Cadillac poor mileage (Page 2/2)
sanderson231 JUN 16, 11:28 AM
The PCM was reworked by Sinister Performance to remove the Pass-Key, change the fan settings and remove the vehicle speed limiter. Otherwise it is a stock tune.

I'm also working on a friend's 4.9. The injectors were from the junk yard and were so corroded they could not be cleaned. I pulled the injectors from my car and tested them on my DIY test stand. Two of them were partially plugged so I sent them off to Mister Injector in Idaho for cleaning. These cleaned up so I put them in my friends car (not quite running). My car got a new set of Bosch injector from Motor Man. The new injectors seemed to give a little more power but no change in mileage.

I checked the base timing with a timing light (AB on ALDL jumpered). The base timing was slightly advanced at 11 BTDC degrees vs the book value of 10 degrees. I took the car for a drive watching the timing an a scan tool. With light throttle or coasting at 60 mph (1900 rpm) the commanded timing was in the high 40s and sometimes pegged out at 50. At WOT the timing dropped back to about 35 degrees. These seem to be what one would expect with a mechanical/vacuum advance distributor.

I can't see or smell any gas so I don't think there is a leak to the great outdoors.

I guess at this point I'll try a new O2 sensor.

I really need to get my friend's car running to use as a comparison. Unfortunately Murphy has been hard at work in my corner of the Fiero world. I got stranded on the road in my 88 3800 S/C and had to have a shop work on the brakes. When I got the car back the brakes still weren't right which I traced to an unusual problem with the master cylinder. I swapped master cylinders and then went to bleed the brakes. When I went to take the left rear wheel off, I discovered that the shop had cross threaded one of the lug nuts and jammed it on with an impact wrench. When I tried to get it off with an impact wrench, the stud spun in the hub. I'm hoping I can reach the back of the stud with a stick welder and tack it to the hub. Otherwise it looks like the only alternative is to drill and a die grinder the lug nut. I'm stopping by the shop today. Should be an interesting conversation.
Raydar JUN 20, 11:14 AM

quote
Originally posted by sanderson231:
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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.



Since you already have your intake, I may be a little late with this, but you should use the steel Allante rocker rails, too. They have to be removed to get the intake off, so it's no extra work.
People have been known to pull studs out of the aluminum rails. It has been suggested that you can drill and tap the hole, and install a bolt that "bites" more threads, but I don't remember the details.

I had to set my base timing to ~20 degrees. (The car had no power when set with the light, so I set it by ear, and then checked it with the light, when I was satisfied. 20 degrees, or thereabouts.)
I thought this was an anomaly - maybe a slipped balancer ring - but I have talked to people regarding two other 4.9/Allante installations, and they had to set theirs to around 20 degrees, as well.

You will also need to import the IAT table from the Allante tune. The Allante IAT is in the upper plenum, and will see much cooler air temps than the 4.9 is used to. (Skipping this is exactly why Allante/4.9s run pig rich.)

The TPS is wired up a bit differently. The Allante wiper in on one end of the plug. The 4.9 wiper is the center pin.
I initially hooked mine up wrong. Made the transmission shift at some really strange times.

All the other wiring matches up perfectly. I think you need to extend the map wiring, but nothing is crossed up or changed. Oh... The 87 and later intake (which is the one you need, to match your heads) does not have EGR. You will need to have it disabled in the tune.

I retained my 4.9 Alternator. The Allante alternator and bracket are different. I think I retained my 4.9 bracket, but it required a bunch of surgery with a cutoff wheel and/or grinding wheel. No welding was required. I supported it from the top bolt with a piece of flat stock, bent appropriately.
sanderson231 JUN 22, 04:34 PM
Raydar,

Thanks for the input on the Allante intake swap. I was puzzled by the IAT table difference. If the MAT reads the correct temperature why does it need correction? However, after educating myself the MAT table seems to be a correction to the MAT reading to account for heat soak causing the MAT sensor to read high. On the 4.9L heat soak seems to be significant. With 100 'F ambient my MAT at idle reads 225 'F. Holding 2500 rpm in park the reading drops to 185 'F. I don't think that this is an actual change in intake air temperature but the fact that higher air flow takes away more of the heat conducted to the MAT sensor from the cylinder head and the MAT reads closer to the actual temperature. The location of the MAT sensor in the Allante intake makes heat soak much less of an issue. Using the 4.9L MAT table with the Allante intake will cause the ECM to think that the air is denser that it really is causing the ECM to add fuel. Maybe so much so that the O2 sensor can't correct the mixture back to 14.7:1.
Raydar JUN 22, 10:19 PM

quote
Originally posted by sanderson231:

Raydar,
...
If the MAT reads the correct temperature why does it need correction? However, after educating myself the MAT table seems to be a correction to the MAT reading to account for heat soak causing the MAT sensor to read high. On the 4.9L heat soak seems to be significant. With 100 'F ambient my MAT at idle reads 225 'F. Holding 2500 rpm in park the reading drops to 185 'F. I don't think that this is an actual change in intake air temperature but the fact that higher air flow takes away more of the heat conducted to the MAT sensor from the cylinder head and the MAT reads closer to the actual temperature. The location of the MAT sensor in the Allante intake makes heat soak much less of an issue. Using the 4.9L MAT table with the Allante intake will cause the ECM to think that the air is denser that it really is causing the ECM to add fuel. Maybe so much so that the O2 sensor can't correct the mixture back to 14.7:1.



Right!
The bottom line is that, no matter what ambient air temp is, the IAT in the 4.9 intake will read significantly higher that the IAT in the Allante upper plenum. Just because of the proximity to the engine itself. Especially due to the EGR passage which is only a few inches away, through the aluminum casting. Since the Allante reading is much colder, the 4.9 ECM will attempt to richen it up, to "accommodate".
Here's the rub... Due to the "cold" temp, it might not even be going into closed loop. (I don't know this for sure. PURE speculation.) If that's the case, the O2 sensor is just along for the ride.
All I can tell you is that mine runs correctly. At a slow cruise, my BLMs hover between 127 and 129. I get low/mid 20s with a Getrag.

[This message has been edited by Raydar (edited 06-22-2022).]