Hi,
Need some advice/confirmation. I have a 1984 Pontiac Trans AM LG4 all stock like it rolled off the factory floor. 90K original miles. I think I have an issue with the TPS. When I start the car cold it idles fine..as soon as it goes into close loop, the rpm's increase to close to 900 rpm (From 700 rpm in open loop) in park. I have a snapon mt2500 and it says cold (not started) the TPS is 1.01v When I start it and it warms up (goes into close loop) it goes to .88 at idle. if i push down on the gas pedal it does choppy readings to 3.80v so that is my guess it is TPS is flaky.

Now I don't know if there is anything wrong other than the IDLE going up a bit and every now and then a slight "Dieseling" when I turn it off (doesn't happen all the time just sometimes so may not be related). The car runs perfect, plenty of power, no other codes, never stalls, doesn't have a rough idle so basically its perfect other than the above so I am a little hesitant changing the TPS since all else is good.

Just looking for thoughts if this is normal operation as I have not had the car that long.

I use 87 octane gas (being an LG4) but I have also tried 93 octane and it still "diesels" sometimes. The high idle cam does kick down. Gas mileage is not horrible at 15/16 mpg.

If TPS could be issue are there any instructions on how to replace?

Thanks

 

Replacing the TPS requires removing the air horn. Not likely that you can do that to an old carb without destroying the gasket. At that point, starts to make more sense to "rebuild" the carb. (totally disassemble, clean in solvent, replace all "wear" parts, reassemble with all new gaskets O-rings etc., restore all adjustments to their correct values in the process)

Don't rely on a scanner to read that voltage. The ECM's data rate is WAY too slow to display it accurately as it changes. Instead, use a DMM, with a sewing needle or something similar to probe the connector: you can easily get that into the cavities and touch the pins, without damaging the wire, the pin, or the connector body. One pin should be ground; one should have constant 5VDC on it with the key on; one should be at around 0.5 - 0.7 at idle depending on things like the idle speed adjustment and the fast idle cam setting being active, smoothly increasing to 4V or so at WOT (3.8 sounds lower than it should be but not unreasonable). If the voltage does not increase and decrease smoothly as the throttle is slowly and smoothly opened and closed (have your assistant use the pedal rather than trying to work the throttle by hand) then it's time for a new one.

Octane "rating" won't materially affect dieseling.

If you decide to disassemble the carb, pay CAREFUL attention to all the adjustments, specifically the lean stop (weird screw thing that holds the mixture control solenoid in), rich stop (thing about the size of a dime in the air horn right above the MCS, and idle air bleed valve (large thing in the air horn near the air cleaner stud). Make a CAREFUL note of where they are set now by CAREFULLY screwing them all the way in while counting the turns, and CAREFULLY put them back to the same # of turns after cleaning, replacing the IAB O-rings, etc.

Note also that in a carb that old with that few miles, there is ALMOST CERTAIN to be a considerable amount of dissimilar-metal electrolysis corrosion in several places. The main one being the metering well plugs. Those are made of some kind of aluminum alloy, spun into large holes in the chinesium casting that were drilled during mfg to allow access to where smaller passages needed to be drilled, then sealed up afterwards by the plugs. These are on the bottom of the fuel bowl, visible when the throttle plate is removed.



Plugs 3 & 4 are the worst offenders: these 2 have vacuum (suction) on the bottom side, and fuel above. They are at the lowest point in the fuel bowl, and therefore ANY moisture that gets into the carb, which it ALWAYS does when the carb sits around unused for awhile and MOST ESPECIALLY with fuel that contains ethanol as virtually ALL fuel does nowadays, ends up there. This sets up a little "battery" due to the 2 different metals being in electrical contact with each other and immersed in the conductive fluid (water), resulting in a small electric current flowing in them. One of the metals (the chinesium in this particular case) erodes, while other metallic ions (gunk) accumulate harmlessly on the other (the aluminum plug). This ALWAYS happens to EVERYBODY: it is INEVITABLE, so please don't bother protesting about "my car doesn't have that", it is as CERTAIN as night following day. The chinesium turns to a white powder, and eventually enough of it erodes that the plugs no longer seal, and fuel begins to be drawn into the engine around the plugs. They MUST be re-sealed, otherwise your gas mileage will continue to be poor, and various other symptoms will present themselves in varying severity and order; long cranking after sitting overnight, sooty idle, poor gas mileage (such as you report), fouled plugs, and various other obnoxious behaviors. The solution is to wire-brush them down to CLEAN NEW VIRGIN metal, STERILIZE them with non-residue solvent such as lacquer thinner, MEK, denatured alcohol, or spray carb/TB cleaner, but NOT "paint thinner", turpentine, mineral spirits, or "brake parts cleaner", epoxy them over with the steel-filled kind of epoxy, and cure the epoxy in a warm (200° or so) oven for a few hours before exposing them to gasoline again.

Plugs 1 & 2 CANNOT leak fuel into the intake no matter what unless the gasket is damaged, so don't bother messing with those. Even though they do indeed often leak, as the ones in the photo clearly have been (not my photo by the way), it is harmless, because the gasket keeps the fuel contained. Plugs 5 & 6 will leak fuel onto the intake manifold so if you see a pile of fuel gunk underneath them it's a SURE sign that the process described above has attacked your carb and #1 & #2 are also failed.

For that matter, if you remove ANY part, such as the screws, and see white powder on them, then you may be sure your carb suffers from this, the same as everybody else's. Some carbs will get it so bad that the needle & seat, where it screws into the fuel bowl, won't seal. Screw threads will strip from this because the metal has disintegrated, which you won't find out until the very end of the reassembly process. I've had to Heli-Coil ALL of the threads in carbs before, because the chinesium they are made of was eroded so bad. Don't be surprised if yours are as bad, it happens to them ALL sooner or later.

 

the readings you are seeing for TPS voltage are not out of line nor unreasonable.

Cold, the choke should keep the throttle slightly open and result in a slightly higher TPS voltage than warm idle. 3.8V at WOT is also close enough.

to second the above, the scanner may erroneously show a non-'smooth' change in TPS voltage as the throttle is opened. opening it slowly should result in a relatively smooth increase in voltage though,.

your dieseling should not be a direct symptom of a failed TPS. TPS voltage not returning to idle specs (at idle) can cause the ecm into holding open the throttle kicker solenoid causing a higher curb idle speed BUT on shut down the kicker should no longer be energized and the throttle should return to curb idle setting.

you can take the airhorn off while the carb is installed and replace the TPS. done that, but probably best to remove the carb to work on it on a bench-lots of little screws and things. then you might as well...

if you need any help, just ask.

 

Thanks SofaKingdom and NAF great info and will try a DMM for readings. I'll let you know if I need any help, if and when I do a rebuild of the carburetor.

 

what is a good epoxy to use for this? I will be starting a rebuild on an E4ME this week. Surely it can't be as simple as using JB Weld. (2 part epoxy... has steel filler.... and is fuel safe...)



also, going to premium octane rating can band-aide dieseling. If your car is a little hard to start when cold out and diesels occasionally at shut-off when hot you could have a choke hanging up. It keeps the throttle blades cracked bleeding in a little bit of fuel & air that then lights off in the hot cylinders. perhaps other fuel leak sources would do the same thing. Higher octane fuel will be more resistant to lighting off so easy and may not diesel.

 

always used JB weld.

be sure to check for play on your primary throttle shafts. with them open look for wiggle front to rear. with the age of the unit it's likely to require bushing. if you need a bushing let me know. I bought a 20 pack of them several years ago and can spare one or two. the throttle side usually only needs one.

a worn throttle bushing will also cause the throttle to hang up slightly open and can cause dieseling.

i did a write up in a thread SEVERAL years ago on re-building this carb. it 'might' be a useful resource.