L98 TPI Transplant Issues

I went out today and did some more work on the truck. There are still no codes in the computer after driving it again today. I did however "pinpoint" more of the rough idle symptom. When the truck first starts up, it idles perfectly fine. It will continue to idle fine for hours in park or neutral. However, when idling in an gear, the engine goes through a "cycle". Example is I pull into the driveway and come to a stop, leaving it in gear. It will idle horribly for about 25 to 30 seconds. Then, all of the sudden, it will smooth out and idle just great for about 20 seconds. It then goes back to "bucking", idling terrible as the engine surges and the truck lurches forward about once a second. This cycle will continue forever or if shifted into Park or Neutral, it will idle fine. Perhaps that can provide more details as to what the issue is with the bad idle?

The no-start is still another issue that I haven't gotten any further on.

I did also go out and check the grounds today. All the measurements between battery (-) and block, (-) and body, and block to body were all about 0. The ECM ground is also good. I did this with the headlights on as well; which they are not dim.  

here's a simple test....

engine off...    grab the throttle lever on the throttle body...    open the throttle about half way...  see if the shaft wiggles around inside the bore????  

why would this effect the idle... the TPS is on the other end of the throttle shaft...  if there is excessive throttle shaft bore wear...  the TPS may return to a slightly different position... this will send a slightly different voltage to the ECM... this will effect the part of the program the computer is in...    the idle program.. or the run program...  this maybe whats causing your unstable idle...   as the ECM thinks you still have your foot on the gas...    or for some reason.. you have closed the throttle farther than is possible so it has to reset the idle voltage it thinks is closed...

since the throttle is farther closed sometimes that others... there may not be enough air around the throttle blades to start the motor...


if for some chance... you had a scan tool...   sitting there with the engine idling..  look at the TPS voltage ... see if the voltage comes back to the EXACT same spot every time you work the throttle...

i would also look at the IAC.. idle air counts...   if the throttle is set properly...  with the engine at operating temp... 195 to 220F... you should have between 20 and 50 on the IAC display on the scan tool..

the IAC is a stepper motor with a range of 0 .. all the way closed...   and 135 to 145 wide open..  since the ECM controls the idle air control motor.. you cannot set this... there are tools to move the idle air control motor .. but when you plug it back in.. the ecm is going to move it ...

the ecm also uses the Idle air motor as a dash pot..   when you bring the RPMs up..  the ecm will command it totally closed for a second..  to see where 0 is... then take it back to somewhere are 100 counts.. while you have the throttle open.. when you close the throttle..  the ecm starts closing the idle air motor to bring the RPMs down slower so super high vacuum is not generated in the intake...   why...  super high vacuum in the intake can pull excess fuel from the just barely opening injectors...

oh sure.. i have seen it happen.. i had a 200SX.. that had a weird problem..  i had the fuel pump routed back to the return line and was running the motor on the carbon clean machine..    the carbon clean machine would shut the machines fuel pump off when the line pressure dropped to 0..   but the cars engine still would idle...  i pulled the quick disconnects and unplugged the fuel rail completely from the carbon clean machine.. and the car would still idle..  the intake manifold vacuum was pulling fuel through the pulsing injectors...   that was really weird...

one more thing...    got a watch that displays seconds...  see if you can count to see how long the engine misfires / stumbles when you drop it back to idle...   the TPI motors were designed to idle at 450 to 500 RPM.. in gear.. they used a special voltage regulator in the alternator.. that when the alternator speed would drop to a selected RPM..  it would stop charging..  this gave the motor time at LOW idle speed to stabilize..
after 2, 5 or 10 seconds.. the alternator would start to charge again..   now.. the alternator does not have any way of talking to the ecm.. but the ecm was programmed to know that the alternator is going to do this..

so just before the programmed in time without charging.. the ecm will back the IAC back a few steps..

if your alternator does not have this low speed no charge built in.. the engine may stumble or stall..

live anywhere near los angeles???   or do you have a scan tool.????

i know this stuff from hand on... and i went to gm tech training in burbank through ac delco...  we sold lot of delco products at the shop i worked at.. and kept a few hundred bucks worth in stock.. that got us in the program..   i think we had to buy 750 a week in delco products.. we did that easily..  i still have some of the early delco parts cabinets from that program..  

Thanks for the suggestion wayne. I'll have to give the throttle a check over. On the topic of the alternator, I did replace it since buying the truck. Is it possible that the replacement one is not the correct type?

Unfortunately I live in Indiana, nowhere near Cali. I'm going to see if a guy at work has a scanner. I had heard he had one but that it was for ODBII and not ODBI. Not sure if that matters or not.  

there are several OBD1 and OBD2 combo scan tools available

Equus - CanOBD 2&1 Tool KitTM  Part Number: 3120


Equus - Code Scanner Part Number: 3140
Scan Tool CanOBD2&1 KitTM


i have also seen OBD1 scan tools on craigs list for cheep.. compared to their original price...

look for snap on MT2500...

there are several actron units.. both new and used that will read these..

most autoXray units will read OBD1...

there are OTC monitor 2000 and monitor 4000 and 4000E units.. along with once in a while a genisis unit..


having an OBD2 scan tool can pay for its self in just a few NON trips to the dealer..  having the OBD1 option makes it truly useable..

since most mechanics charge 40 to 90 bucks to read the codes.. and dealerships get up to 140..

only thing.. the real professional scan tools start at about 2 grand.. for a genisis.. and work their way up to 8 grand for a new snap on modis unit..  but when you use it 5 to 10 times a day.. they pay for them selfs also...  

I was looking into getting a cable for my computer and then using either some trial or free software to look at the ECM. This is what I'm looking at: http://www.aldlcable.com/sc/details.asp?item=aldlobd1u