I have done some research and cannot narrow down my specific issue. My current TBI stock set up is idling a little bit off and throttle response, at least in park, has lag. I checked the TPS sensor voltage at idle and its around 1.16v, non adjustable type TPS. I pulled the TPS off the throttle body with the harness plugged in and TPS voltage does not drop. I can open it all the way and get it to about 4.5v as it should. So, I purchased a new TPS and installed that one to the harness and tested. Well, although the volts are lower at idle or closed setting, it still sits at .93v. At this point I am not sure if I have 2 bad TPS sensors, possible but not likely, or something else I am missing. The process used to test the TPS is with DVM on the 20 VDC setting probing wire A and grounding wire B. Wire C has constant 5 volts with key on as it should. I am not looking to keep buying sensors, but there is definitely something going on with the TPS here.

 

After doing more research and coming across the 1991 camaro service manual, really good source of info, it would seem that for the TPS sensor anything under 1.2 volts at idle ie fully closed, is within normal range. So for the time being, I will leave the TPS sensor alone and move on to testing the MAP and IAC sensor and look at my timing again. A lot of people say the TBI cars like a little bit advanced, so I will try it at 0 first and slowly increase timing to see how it reacts and try to find the happy spot.

 

First of all what make & year is your car and what engine does it have?
In my experience most manufacturers use around .5 to .6 some volts from the tps for idle. This is due to ecm sends a 5v reference voltage to the tps sensor and if it sees below .5 volts or above 4.5 volts to ignore the signal and go to default idle and set a code. On my 89 TPI 350 the idle tps voltage should be .54 volt (+or- .07v).
A engine that idles high can be caused by various items (vacuum leaks, throttle plate stop screw mis-adjusted, IAC gummed up and sticking, poor electrical connections causing resistance or shorts to higher voltage.

 

It is a 305 TBI in stock form. I have been slowly piecing the car back together. There are no vacuum leaks that I have detected, Vacuum at idle is at 20mmhg and holds steady. All the sensors on the car are new, I think with the exception of maybe the ICM and the distributor itself. Most of the research I saw stated the .54 to .62 at idle is where the TPS should sit at idle. Being that I have now tested 3 sensors, lowest being .93 and highest being 1.55v at idle, I want to go back and check ground wiring but the actual service manual states that for the TPS sensor anything under 1.25 volts at idle ie throttle blades closed is within range and the computer sees that as closed.

I need to check wiring again for the TPS, confirm timing as well and make any adjustments needed. If I still have the slight idle surge and poor throttle response I will move on to MAP and ICM testing.

 

Yes, be sure that all engine grounds are good (especially the ones on the back of each head). Also see that the 5v reference voltage from ecm is actually around 5v and not higher (if higher the ecm may be the problem). Also even though the manual says anything under 1.25v TPS is idle your losing that initial .5v to .75v of increase when stepping on the throttle and maybe contributing to your stumble.

 

Don't the TBI cars auto zero out the TPS on each start up like the 1990+ SD TPI cars?

 

The reference voltage is right around 5 volts and ground wire tests good as well. I need to double check all the other grounds as well, like I said I am piecing this car back together. There are always surprises hidden with 35 year old cars that were hacked up. I ordered an ALDL cable so that I can get better info from the sensors using tunerpro rt. Should give me a better idea of what is going on with everything.

 

Hi idle at an elevated TPS reading . If it were me, I'd check the minimum idle screw and back it out a full turn. Any way you can check your IAC counts and visually inspect that its traveling in and out freely?

 

I have adjusted the idle screw to get it to idle better, not sure exactly where it’s idling st because the tach is not correct and I don’t have a aldl cable yet to data log, coming this week. After setting the timing at 4 btdc it seems to be idling smooth but I’m still not thrilled with the laggy throttle response. Again, all this testing is at idle with the car in park, haven’t had the car on the road yet to test it under load.

 

TPS at idle should be between .5V and .65V and of course that's gonna change when you adjust the curb idle screw. Yours reading .93V makes me think you've got the curb idle screw turned too far in. There is two ways to adjust curb idle. One is to jump pins A&B at the ALDL and turn the key on, that will run the IAC in against its seat then unplug the IAC connector and pull the jumper out of the ALDL. Start the motor and using a tach set the curb idle 100-125 RPM's below what it is supposed to idle at. Shut the motor off, plug the IAC back in and remove the ground from the battery to clear the IAC code it set when you unplugged it. Turn the key on, wait a few seconds then turn the key off. That resets the initial IAC park position. Wait ten seconds and fire it up, you should be good to go. The second method and the one I prefer is to have whatever scanning hardware you have hooked up with the motor running and adjust the curb idle screw until you have between 15 and 30 IAC counts, I shoot for 20. Some people say TPS voltage should be .5V with the throttle closed, some say .6V. On my truck it likes .66V. When I set mine to .5V it had a dead spot at the bottom so I kept creeping up the TPS until the dead spot went away and that was .66V. You can use a dremel tool or jewelers file to elongate the screw holes to give your TPS some adjustability.

 

Oh yeah, no matter what your TPS voltage is the ECM will reset the TP% back to 0 when you shut the key off.

 

The lowest voltage the TPS reads is .93, that is with the TPS removed from the throttle body, plugged in and tested with volt meter.

 

Sounds like you got a bad TPS. Here's a tip! Don't buy the cheapest electronic parts they offer and try to determine the origin of manufacturer before you throw your money away. Most of this stuff is chinesium and many of the auto related electrical parts are made in the toy plants. Japanese, Mexican, South American and Portuguese parts are still decent quality and sad but true, better than American. German parts are pure garbage right now too.

 

How are you testing the TPS voltage? The proper way is back probe the sensor ground(not chassis ground)and the sensor wire. Should be black or black with a white tracer and dark blue.

 

I have tested it both ways, lowest I get is.93. I have tested 3 different TPS sensors at this point, none of which read close to .45-.6 range.

 

Is your reference voltage 5V?

 

yes reference voltage is right around 5v. I just got an aldl cable to datalog. I am going to do some datalogging tomorrow to see what all the sensors are detecting.

 

TunerPro? If so your data dashes will give your TPS voltage, percent and IAC counts.

 

I am using winaldl for datalogging, should give me all the same sensor parameters as tunerpro. I will be look at it tonight when I get home.

 

 

Just to update, after running into some other issues and digging deeper into the rabbit hole that this car is, I have determined that grounding straps are missing. From what I have found, it looks like there is only one ground strap/wire on the car as of now and it would seem it is not hooked directly to the block, but instead to the AC compressor housing. I have ordered new grounding straps and will add 2 more, one from head to firewall and 1 from block to chassis as well as move the 1 ground wire to the block itself. I know that this will surely cause issues and hoping it clears some things up.

 

Got back to working on the car after about a month. When I set timing and I advance to about 8 degrees that throttle response is better versus setting it at 0 or 2*. However, even at 8* BTDC there is still lag with quick stabs of the throttle, so I said ok let's try something. If 8* BTDC improves throttle response, lets keeping moving the distributor counter clockwise until I get a proper throttle response. The result, I finally got it to a point where the throttle response is crisp and but for timing, I couldn't even tell you what it's at because the mark is literally at the center of the harmonic balancer. The other issue is that idle is now not settled, slight surge in park and in Drive it surges more. Tried adjusting idle screw but had to open the throttle blades to the point that the idle in park was probably sitting around 900 (guessing by sound, because tach doesn't read right). I am thinking that maybe the distributor is not set to #1 TDC. I tried moving all the plugs over one spot to the left to get the #1 plug wire pointing at #1 cylinder but as I suspected that car won't start. It seems to me the distributor is clocked wrong (I did not set the distributor, it was set that way when I got the car). Any thoughts, before I go and pull the distributor and try to reset it to TDC #1. I included a picture of the current cap orientation and the yellow tape is the #8 wire, which almost lines up with the #1 Cylinder. Yes, the timing was adjusted and set with the Brown wire disconnected.

 

Invest in a moates cable, laptop and tunerpro RT. Sounds like you’re fighting a vacuum leak.

with the TBI, you can read MAP. Should be around 26 Kpa - 29kpa at idle with IAC around 10 - 20 steps. Anything higher will show up in poor throttle response, surging idle, etc. I run my TBI at 8• advanced and runs super strong.

you really should confirm vacuum first before doing anything else.

 

I have confirmed vacuum and it sits right around 20mmgh with a vacuum gauge, but I will check again. I have a cable and winaldl to datalog as well, will need to run a datalog and see what it tells me now with the timing.

update- just confirmed vacuum at the MAP port and the large main port PCV valve at the throttle body, Sits at about 20mmgh but obviously drop a little when it surges. Something to note, when I set the timing at the timing mark to anywhere from 0-8* BTDC, the idle is steady, car starts right up, engine temps stay cooler but throttle response is less than good. When I continue to move the distributor to a point where throttle response is great, idle then surges a little, engine temps get hotter faster and engine start up isnt as fast (not slow or problematic at all just takes a couple cranks).

 

In a previous post, you mentioned the ICM had been replaced. Did you replace with a used oe or an aftermarket? I chased a throttle response issue on my mercruiser powered boat in the same manner you are, only to discover the aftermarket ignition module (gm small cap style) was the issue. The timing curve was apparently wrong. Installing a used oe fixed my issue

 

The ICM is the one that was in the car when I got it, I have not replaced it. I am trying to avoid just buying parts and hoping that's the fix, but it might be my next purchase. The one that is in the car seems to be an oem one, ie GM. I would have to remove the cap again and confirm. I have changed the plugs, wires, cap, rotor, TPS, IAC, MAP sensor thus far. All new gaskets on the throttle body as well. vacuum doesn't seem to be the issue, timing does or an ignition item more specifically. Only parts i haven't changed at this point is the ICM, Distibutor and coil.

 

Right now #8 is where #1 should be but if the dist. is clocked wrong that's probably where it needs to be. The TBI dist. doesn't care how it's clocked as long as the wires are clocked along with it. You shouldn't be running more than 10deg. initial. I static time my dist.'s by rolling the engine with a breaker bar up to whatever advance I'm dialing in at #1 firing then align the points of the reluctor with the points on the pickup coil. Remember to come up to your timing point in the normal engine rotation to remove any timing chain slop and hold a little counter clockwise pressure on the dist. shaft to remove timing gear lash while lining up the points. You will be dead nuts if you do it right and maybe +/- 2deg. if you hurry the procedure. I don't even break out my timing light anymore to set initial timing, only to make sure the timing on the scanner is the same at the crank. You really need scanning software to avoid buying parts you don't need and to properly diagnose what your motor is doing. If you do need electronic parts you're best off going to the salvage yard and getting original factory parts than buying even so called GM replacements, most of it is pure chinese garbage now.

 

That sounds like the best, most logical step to take. I don’t like how the timing mark doesn’t jive with the way the engine runs. I’ll follow the procedures you laid out and see what it shows me and then move on from there.