TPS at low throttle, GM EFI 4cyl - LotusTalk - The Lotus Cars Community
 
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Old 06-09-2013, 12:10 PM   #1 (permalink)
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TPS at low throttle, GM EFI 4cyl

After putting the FiveO injectors in my S4s and increasing the base idle to compensate, I ran through the other numbers with Frescan. I notice that the TPS shows 0% for at least the first 5% to 10% of travel. You can see that in both percentage reading, and the actual TPS voltage. I can slowly raise the engine speed with no load to 2,000 rpm with no change in TPS voltage.

The injectors and base idle have eliminated 90% of the off idle stumble, but there's just a tiny bit left. I'm thinking the TPS might need to be replaced, that the area where it lives around idle gets most of the use, and is not very responsive. The TPS works fine if you crack the throttle in a big way. Before replacing it, I'd like to know if that's the way others are - that the TPS does not sense movement for the first 5% or so of movement?

I'll probably change it anyway to be sure. Crystal Princess cross reference shows GM part # 17106680, for Quad 4. Can anyone confirm that?
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Old 06-09-2013, 12:33 PM   #2 (permalink)
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You should see movement straight away, every 1 A-D count is (very) approximately 0.4% of throttle sensor movement. See the attached graph showing movement straight away (where TPS is green, rpm is black, MAP is red and mph is blue).

What are the A-D counts during the first 5% of movement?
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Old 06-09-2013, 02:46 PM   #3 (permalink)
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Try adjusting if possible, most will read 5% with the throttle closed because they never are actually completely closed.
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Old 06-10-2013, 12:37 AM   #4 (permalink)
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The throttle percentage starts at 0%, this is because it is based upon the relative throttle position (i.e. the current throttle position value minus the closed throttle position value).
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Old 06-10-2013, 05:52 AM   #5 (permalink)
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Does your tps read 100% at wot?
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Old 06-10-2013, 09:10 AM   #6 (permalink)
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Quote:
Originally Posted by CarSalesman View Post
After putting the FiveO injectors in my S4s and increasing the base idle to compensate, I ran through the other numbers with Frescan. I notice that the TPS shows 0% for at least the first 5% to 10% of travel. You can see that in both percentage reading, and the actual TPS voltage. I can slowly raise the engine speed with no load to 2,000 rpm with no change in TPS voltage.

The injectors and base idle have eliminated 90% of the off idle stumble, but there's just a tiny bit left. I'm thinking the TPS might need to be replaced, that the area where it lives around idle gets most of the use, and is not very responsive. The TPS works fine if you crack the throttle in a big way. Before replacing it, I'd like to know if that's the way others are - that the TPS does not sense movement for the first 5% or so of movement?

I'll probably change it anyway to be sure. Crystal Princess cross reference shows GM part # 17106680, for Quad 4. Can anyone confirm that?

How did you increase your "base idle"?

There should be a small amount of slack in the cable when the engine is warm so that the cable isn't pulled too tight with the growth of the hot engine. So if you are referring to the fact that you have to push the pedal a bit to see the TPS change in freescan, then that is correct.
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Old 06-12-2013, 06:04 PM   #7 (permalink)
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Got this done today. Bad news is the TPS GM part #17106680 that I had ready is not correct. As discussed some other places, 17106680 operates in the opposite direction. The correct part number for GM injection 4 cyl Esprit is 17106681. Only the last number is different. I don't know who maintains the various online Esprit parts interchange lists, but if that's you please update with the correct number for the next guy.

Good news is that both are very common, available at most auto parts stores, so it did not hold up the job. The TPS is prrobably the cheapest thing on the car.

The other good news is that the replacing the TPS fixed many problems on my car. I have been struggling with an abrupt off idle that jerks the car when you are trying to feather the throttle in a turn. I did several other things earlier in search of that problem, including replacing the injectors (which it ultimately did need). The TPS replacement virtually eliminated that problem, at least to the extent you could expect from a 1980s design computer system.

The other big thing that happened is that the car really woke up on the top end. Apparently my secondary injectors were not actually firing due to the TPS not giving a high enough signal. Rode home tonight, the car is now an absolute rocket as it always should have been. When the secondary injectors hit, its like the afterburner on a jet. I get the normal Esprit kick when the turbo comes in at the bottom, then another kick at the top. I always thought this particular car was a bit doggy on the top end, but now it accelerates so fast I have to watch out for the rev limiter in the first three gears.

I've always been amazed how the Esprit responds to engine and chassis being tuned just right. You could say the same about any car, but on the Esprit, very small changes make a big difference. Last week I commented here how a 1/2 degree change in front camber changed the character of the car. I've been lucky enough to drive many of these cars over the last 30 years, at least thirty different cars. Only three of those I kept for personal use, the rest were for resale or customer service in the 1980s/1990s. Few of them are set up right because the owners don't realize what's missing, but if you're persistent they all can deliver that Lotus feel. If the previous owner of my car drove it now, he would not recognize it.
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Old 06-12-2013, 06:09 PM   #8 (permalink)
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Very interesting, I've been logging ecu's using freescan and have yet to see my secondary injectors open. I'm going to take a look at my TPS trace and see if it exhibits the same behavior. My off-idle stumble was cured by injectors and it certainly has plenty of power, but hey we can always use more
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Old 06-12-2013, 07:42 PM   #9 (permalink)
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Just check the throttle percentage indication with key on, engine off. At full throttle I am reading 96%. I think the secondaries kick in at 90%, not sure on that number.
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Old 06-12-2013, 10:21 PM   #10 (permalink)
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The secondary injectors are not enabled by a minimum throttle position. There are however minimum engine speed, MAP and battery voltage requirements.

Their use can also be disabled due to error codes 21 or 22 being set and if any of the speed limiter, DFCO, decel enleanment or boost limit flags are set. They will also be disabled if the calculated dutycycle is less than a small amount (e.g. ~1.5% stock SE code).
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