technoweenie Posted February 1, 2012 Share Posted February 1, 2012 1990 Coupe. Symptom is an idle speed that hunts and is often high (1000-1500rpm). Throttle body has been cleaned and IAC replaced. Here's why I suspect the throttle position sensor (TPS):If I go into diagnostic mode and monitor Ed01 (TPS voltage) it fluctuates greatly. With engine off, foot off accelerator, it varies between 0.40-0.46 (not bad). But when the engine is started it jumps around from .48-1.04.I'm guessing engine vibration is causing the wiper within the TPS to move around on a worn spot. The hunting rpm would then be a result of the ecm falsely thinking the throttle has been moved and commands the IAC to react accordingly. If I monitor Ed22 (IAC position) it is indeed rising and lowering as the TPS voltage fluctuates.Am I on the right track here? The TPS looks a bit tricky to replace but if that's what it takes to get a stable TPS voltage at idle I'll tackle it. Thanks in advance for your feedback. Link to comment Share on other sites More sharing options...
Ronnie Posted February 1, 2012 Share Posted February 1, 2012 With the engine off does the TPS reading increase smooth as you slowly press the accelerator? If it is erratic you likely have a defective TPS. I have the correct TPS listed here:The Reatta Store*-*ReattaOwner.comLook on the second page in the Computers and Sensors category. The TPS isn't hard to change. Link to comment Share on other sites More sharing options...
wws944 Posted February 1, 2012 Share Posted February 1, 2012 Does it hunt when the engine is cold, hot, or both? Link to comment Share on other sites More sharing options...
padgett Posted February 1, 2012 Share Posted February 1, 2012 Is the rpm going up with the IAC or is there a lag ? (IAC starts down before RPM peaks & verse the vice).Keep in mind that the IAC reading is a command and not a position. Link to comment Share on other sites More sharing options...
technoweenie Posted February 1, 2012 Author Share Posted February 1, 2012 Ronnie, with engine off TPS reading does increase smoothly as I slowly press the accelerator. The initial value, however, is not the same depending on how warm the engine is. If I run the test cold the reading is often in spec (around 0.40 volts). If I warm up the engine and then turn it off, the TPS still increases smoothly as I slowly press the accelerator but the initial value is way too high (0.78-1.00 volts).wws944, it hunts whether the engine is cold or hot. However, it's not as apparent when cold since normal idle speed cold starts higher to begin with. Cold start idle is initially stable at 1050 rpm. Then as engine warms up the idle speed slowly drops to 900 rpm and then drifts upwards to 1500 rpm, back down to 1000, then it's anyone's guess where it will go. Link to comment Share on other sites More sharing options...
padgett Posted February 1, 2012 Share Posted February 1, 2012 When the tps is high (warm), see if you can push the throttle further closed by hand, may be that the linkage is binding hot and that would confuse everything ese. Link to comment Share on other sites More sharing options...
technoweenie Posted February 1, 2012 Author Share Posted February 1, 2012 padgett, the rpm is going up with the IAC, there is no lag.If I understand the operation, that suggests the emc is causing these changes in rpm by commanding the IAC to do so. Am I correct?I just ran another test. Engine was cool, not cold (60 degrees C). TPS reading was 0.44 volts and the rpm was 760-780. I never touched the accelerator. As I was cycling thru the diagnostic readouts to get to Ed22 to see what the IAC reading was, the rpm was already starting to increase. I went back to Ed01 to look at TPS to make sure it was still at 0.44. It wasn't. It was bouncing around between 0.60-0.80. Link to comment Share on other sites More sharing options...
technoweenie Posted February 1, 2012 Author Share Posted February 1, 2012 padgett, you're reading my mind about pushing the throttle closed by hand. Tried that, didn't make any difference. I also checked to make sure none of the cables were so tight that heat expansion might lift the throttle off stop. Nope, I can rock the shaft that the cables are connected to ever so slightly (indicating at least some play). Sigh. Link to comment Share on other sites More sharing options...
Guest WEB 38 Posted February 1, 2012 Share Posted February 1, 2012 Im having the same problem with an 86 6cly.Fiero I will be watching for your results to the problem. It has been driving me nuts. Bill Link to comment Share on other sites More sharing options...
Guest Mc_Reatta Posted February 1, 2012 Share Posted February 1, 2012 Have you unplugged the connector and cleaned the contacts?Do you have an assistant that can relay the TPS readings while you try tapping the TPS lightly with a screwdriver of something similar like the MAP test? Might also tap on the ECM while looking at the TPS reading. These can be done with engine off.Do you have a DVM?ECM is supposed to supply a +5 volt reference voltage on the TPS via the gray wire on pin A measured against ground on the black wire in pin C.This voltage should remain stable no matter what.The voltage you see in diagnostics (ED01) is the voltage coming off the wiper via the dark blue wire at pin B. This should only change in reference to the position of the throttle not heat or vibration etc. The only hard part to replacing the TPS is getting at the 2 screws that hold it in, particularly the bottom rear one. Need to move some stuff out of the way to get a straight at them with a screwdriver. Small hands help too. Take car not to drop them as with the other screws they are non magnetic so hard to recover. Link to comment Share on other sites More sharing options...
Ronnie Posted February 1, 2012 Share Posted February 1, 2012 I would put my money on the TPS being bad from what you describe. I think the TPS is a just variable resistor that has a metal slider inside that changes resistance in the circuit as it slides along a coiled resistance wire. It's possible that the metal slider contacts the winding at a different spot, that could be dirty, when the engine warms up due to heat expansion. The easiest way to diagnose is to try another TPS that is known to be in good condition. Link to comment Share on other sites More sharing options...
technoweenie Posted February 2, 2012 Author Share Posted February 2, 2012 Ronnie, I like your idea of substitution. Here's what I'm thinking. Before removing the existing TPS, simply plug a new one into the harness. With key on, engine not running, I'd move the lever to achieve a valid TPS voltage corresponding to idle, e.g., 0.40 volts displayed in Ed01. This would mimic a fixed TPS position/value that isn't going to change due to vibration, binding or temperature.I'd then start the engine and observe what happens. If after the engine warms up the idle speed settles to a normal and stable rpm then I'd be confident that it is indeed the TPS. If, however, I still have rpm hunting we can safely say it's not the TPS.Should be an interesting experiment. Stay tuned. This is tomorrow's project. Link to comment Share on other sites More sharing options...
wws944 Posted February 2, 2012 Share Posted February 2, 2012 When cold, the computer is running in 'open loop' and not reading all the sensors. So things like vacuum leaks and leaky FPRs (which let in unmetered air/fuel) don't affect things as much as when things start warming up and the computer goes into 'closed loop' mode.When I first picked up my Reatta 2.5 years ago, it had a hunting problem as well. Main culprit was the FPR leaking a bit of gas through the vacuum line into the intake. Was causing hunting/stalls when warm. Even some backfiring that was causing a vacuum hose to leak. An easy FPR test is to pull the vacuum hose off and check for gas drops in it. If there is any hint of gas in it, the FPR is bad. Link to comment Share on other sites More sharing options...
technoweenie Posted February 2, 2012 Author Share Posted February 2, 2012 Good news, bad news. First the good. Instead of plugging in a new TPS, and being the electrical geek that I am, I created a resistor network (voltage divider) to simulate a TPS. I plugged that into the harness. TPS voltage was 0.46 (both by measurement with a multimeter and as displayed by Ed01).I held my breath and started the engine (cold). The idle was stable around 1000 rpm. When the engine warmed up and went closed loop the idle dropped down to 750 rpm. So far so good. 5 minutes later the idle was still stable. No hunting whatsoever. Outstanding. The experiment was a success. If the ecm sees a stable TPS voltage the engine idles just as should. That definitively points to the old TPS being erratic.As further confimation, I turned off the engine and plugged the harness back into the old TPS. I restarted the engine and the idle speed hunting resumed.Now for the bad news. I cannot get the old TPS off to replace it! How can two little screws be so stubborn. A half hour of fighting with them and I've got skinned knuckles but haven't been able to break either of them loose. I haven't given up but thought the followers of this thread would appreciate the update. Link to comment Share on other sites More sharing options...
Guest Mc_Reatta Posted February 2, 2012 Share Posted February 2, 2012 (edited) Best way to remove the TPS I've found is to disconnect and prop out of the way the upper radiator hose and the coolant line to the throttle body, then use a Phillips head bit that fits the screw head really well, and a ratchet driver like the get up in the photo. One finger presses the bit snugly into the screw head, and the other hand operates the ratchet to crack the screws loose. Then a stubby screw driver can be used to take the screw all the way out or just freehand the bit being careful not to drop the screws into the nether region. Edited February 2, 2012 by Mc_Reatta (see edit history) Link to comment Share on other sites More sharing options...
padgett Posted February 2, 2012 Share Posted February 2, 2012 Dose with favorite penetrant, wit 15 minutes, and strike sharply with a hammer, then wiggle. Link to comment Share on other sites More sharing options...
DAVES89 Posted February 3, 2012 Share Posted February 3, 2012 Good news, bad news. First the good. Instead of plugging in a new TPS, and being the electrical geek that I am, I created a resistor network (voltage divider) to simulate a TPS. I plugged that into the harness. TPS voltage was 0.46 (both by measurement with a multimeter and as displayed by Ed01).I held my breath and started the engine (cold). The idle was stable around 1000 rpm. When the engine warmed up and went closed loop the idle dropped down to 750 rpm. So far so good. 5 minutes later the idle was still stable. No hunting whatsoever. Outstanding. The experiment was a success. If the ecm sees a stable TPS voltage the engine idles just as should. That definitively points to the old TPS being erratic.As further confimation, I turned off the engine and plugged the harness back into the old TPS. I restarted the engine and the idle speed hunting resumed.Now for the bad news. I cannot get the old TPS off to replace it! How can two little screws be so stubborn. A half hour of fighting with them and I've got skinned knuckles but haven't been able to break either of them loose. I haven't given up but thought the followers of this thread would appreciate the update.Why don't you do a tutorial on how to build your TPS tester? If not I would be interested in buying one. PM me please. Link to comment Share on other sites More sharing options...
Ronnie Posted February 3, 2012 Share Posted February 3, 2012 A tutorial on how to build would be great. It would be a really nice tester to have. I would be happy to post the tutorial on my website. Link to comment Share on other sites More sharing options...
technoweenie Posted February 3, 2012 Author Share Posted February 3, 2012 Tada! I am one happy camper. I got the new TPS installed this morning and the engine purrs like a kitten. It now idles smoothly at 750 rpm and no longer hunts.Thanks to all that contributed help and advice. The least I can do is return the favor by writing up my trick for the resistor network I used to verify it was the TPS causing the problem. I'll make that this weekend's project. Link to comment Share on other sites More sharing options...
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