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STR 4.2 S/C: Throttle "dead spot" at ~15-25% throttle position at 80-120kph speed

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Old 06-04-2017, 02:59 PM
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Default STR 4.2 S/C: Throttle "dead spot" at ~15-25% throttle position at 80-120kph speed
Hey! I have a S-Type R, 4.2 Supercharged, MY 2003.

I have now repeated the same annoying problem at maybe around 15-25% gas (throttle): The rpm revs up as supposed, but then both rpm and speed remain the same, the car doesnt accelerate. As if there was a dead spot on the gas pedal / throttle position. Also, might relate or not, to the problem, at around 80km/h speed at flat throttle I have a small RPM fluctuation; I dont know if this is just engine rpm's or the charger, but rpm's move up&down constantly about 100rpm. You can hear it and see it on the gauge.

Any information, experience or educated guesses what might cause this? (Or these)

Seems that the car is (at least partially) responding to throttle, since the rpm's rev up, but then revs remain the same and car doesnt accelerate at all. I need to either lift up or increase throttle to speed up. Even lifting up increases the speed.

This happens at around 80-120 km/h speeds, whan I slightly increase throttle to pass other cars or to slightly increase speed for any other reason. Of course I could kick it down and pass like a maniac, but what I'd really love to have is a fix to this problem

I have an OBD reader and software, so even a hint of what figures I should monitor would help me.
 
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Old 06-04-2017, 03:02 PM
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Start with fuel trims in case you've an air leak - lots of threads to see what to do.
 
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Old 06-04-2017, 03:14 PM
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Originally Posted by JagV8
Start with fuel trims in case you've an air leak - lots of threads to see what to do.
In case you happen to have information what the trim values should look like, I'd love to have the info. If you dont have an easy access to it, never mind, I will start looking

(Already started a search on the forums, but nothing yet).

It's late and I've had a couple beers, so I will post some OBD data about the fuel trims tomorrow. Since I'll be driving 15 km to work tomorrow, that should provide at least some data.
 
Last edited by Stounedi; 06-04-2017 at 03:26 PM.
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Old 06-04-2017, 03:26 PM
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See the threads.
 
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Old 06-04-2017, 06:05 PM
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I had somewhat similar symptoms but at lower rpm's on my non-SC 4.2. May not be the same problem as you never mentioned going into limp mode.

My car drove me crazy for 6 months by going into limp modes at various times and would usually (but not always) throw a P2135 code on OBD reader. It turned out to be a randomly faulty TPS unit. I could never get the bad unit to fail on the bench but that may be that I only had a digital multimeter for testing purposes. Good luck.
 
Last edited by S Daddy; 06-04-2017 at 06:12 PM. Reason: adding to whether comments were applicable to his problem.
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Old 06-05-2017, 01:34 AM
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If it's consistent and if (big if) TPS-related you can just watch it via OBD and see it within moments.

Throws codes, of course, as you had. That is because there are two actual sensors which are cross-checked. So dead spots without codes isn't a TPS issue...
 
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Old 06-05-2017, 02:37 AM
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Short term fuel trims seem to go up & down, upto +-20%
Long term trims stay on the positive, upto +10%
No error codes or faults.

Picture 1 is soon after starting the car, so engine is cold.
Second picture, the engine has warmed up. Less FT changes, but still upto +10% on both STFT and LTFT.

I'll drive a longer trip later and get more accurate values (on long term)

 
Last edited by Stounedi; 06-05-2017 at 03:44 AM.
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Old 06-05-2017, 04:07 AM
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10% is too much. Investigate more - be SURE to read detailed existing threads.
 
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Old 06-05-2017, 09:16 AM
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Ok, I have taken some more accurate readings from semi cold engine and warm engine, both idle and ~2500rpm.

I will be able to drive the car later so I will log actual driving values too.

I have been reading the previous threads, and I think these values might indicate a vacuum leak?

Semi-cold idle
Spoiler
 
Semi-cold ~2500rpm
Spoiler
 
Warm engine idle
Spoiler
 
Warm engine 2500rpm
Spoiler
 
I actually started logging when I drove home from work, but when I stopped the car the software lost the log ... So I can provide more details about actual driving conditions later.

I got some butane so I might be checking for any leaks by spraying the gas around and see if anything reacts.


EDIT:
After startup, semi-cold engine, max 50kph speed
Spoiler
 

80-100kph speed, sport mode OFF
Spoiler
 

Slow down from 100kph to city speeds (around 40kph)
Spoiler
 

Sport mode on, at full throttle launches STFT went to +25% - But not at every launch, at some launches it remained 0. Might be just coincidence, but when I started using manual selection the trim remained at 0 all the time and car launched better. No fault codes, even though +25% trims...
Spoiler
 
Now I should have enough data. I will start digging to the threads... I dont believe it is any kind of vacuum leak... Strange is that at some launches trim remained at 0, and at some launches went to +25% - also it made a difference how the car accelerates...
 
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Old 06-05-2017, 02:15 PM
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Watch this one
https://youtu.be/oRX2V6_a3dc
 
Last edited by Ducmon; 06-05-2017 at 02:18 PM.
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Old 06-05-2017, 02:32 PM
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Originally Posted by Ducmon
Thanks

Next I'm gonna plow through all the threads in the forum (again), to see if anyone else has had close enough same symptoms.
 
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Old 06-06-2017, 11:28 AM
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If you have a vacuum leak you should have a code?
Have you checked for codes?

The RPM change could also be from a failing TQ lock up clutch.
.
.
.
 
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Old 06-06-2017, 12:39 PM
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Originally Posted by clubairth1
If you have a vacuum leak you should have a code?
Have you checked for codes?

The RPM change could also be from a failing TQ lock up clutch.
.
.
.
No codes.

EDIT: I started to think about it, and I dont think the tq clutch would cause elevated LTFT values. A vacuum leak might explain both symptoms.
I am not sure what is exactly normal LTFT on idle, but my warm idle is steady +5,5% on both banks and elevated rpm (2500) changes in the range of 0 - +7% - Which from my understanding from all the other threads is too high.

To be honest I havent had time to check leaks further under the hood.
 
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Old 06-06-2017, 03:48 PM
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Watch this video.

https://m.youtube.com/#/watch?v=5WnM_NsOtd8
about 24 mins will show you if it is an air leak

go to part 2 to see if fuel pump.

and here to see if it is the MAF
https://youtu.be/Hmt_LNJ9GkI
 
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Old 06-11-2017, 07:03 AM
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Ok, finally got time to do a better check on the car. Checked engine bay from above, since I cant lift the car now.

What I found was two connectors that have some movement. I dont know if this is normal.

First at intake tubing.

Second at PCV valve.


But then I found something really interesting. Behind the engine there was a hose completely disconnected. I reconnected it and idle LTFT values got to -1.6% - 0% values - down from earlier 5.5%

2500rpm LTFT values still range form +5.5 to 7.0% so this did not solve all the problems - but some, yes.

What is this hose?
Also, right below the hose + connector (marked by red circle) is two bolt holes / locations. Nothing attached to them. I was curious what are they for. (Bigger on visible in the photo).

Photo of the hose and its location:


I try to document my hunt just in case someone else is struggling with same issues.
 
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Old 06-11-2017, 08:07 AM
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Yes those connections will rotate. If you take them apart you will see they are O-ringed sealed.


Something is not right?

You can't have those way out of spec fuel trims and NOT get a vacuum leak code?

How are you reading codes?
.
.
.
 
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Old 06-11-2017, 09:17 AM
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There is no OBD errors. I get high LTFT values mostly at light throttle (ie cruising on a highway). Accelerating harder or idle everything seems ok. I use OBD2 reader and can pull out the standard P1111 (all tests complete). When I had an ABS sensor fault I could get that code ok with this reader and software.

I've purchased a workshop reader and software but they are coming in 2 weeks.
 
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Old 06-12-2017, 02:58 PM
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Do you have the ability to produce graphs of the ltft and stft. If so you can try to use brake fluid and spray that in the engine bay on the pipe work with the engine idling. If there is air leak they will change immediately. I have found that this method is very sensitive and can show where the leak is.

If this does not work then you can try a diagnostic smoke test.
Here is how to make one that works with incense sticks, you can use one of them tire pumps that work from cigar lighter socket
https://youtu.be/2U5kFib1WxE
 
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Old 06-12-2017, 03:58 PM
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Originally Posted by Stounedi
There is no OBD errors. I get high LTFT values mostly at light throttle (ie cruising on a highway). Accelerating harder or idle everything seems ok. I use OBD2 reader and can pull out the standard P1111 (all tests complete). When I had an ABS sensor fault I could get that code ok with this reader and software.

I've purchased a workshop reader and software but they are coming in 2 weeks.
When you have an air leak you will get higher readings when at lower revs. When you are accelerating you are pulling in more air in so the air leak is relatively smaller.
but it could also be your MAF is dirty watch this video
https://m.youtube.com/#/watch?list=PLAFYVCyenqcrUtzoyk65FMfo9pxhK6Vay&v=CE KjjXNLTjk
 
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Old 12-22-2017, 10:20 PM
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Originally Posted by Stounedi
Ok, finally got time to do a better check on the car. Checked engine bay from above, since I cant lift the car now.

What I found was two connectors that have some movement. I dont know if this is normal.

First at intake tubing.

Second at PCV valve.


But then I found something really interesting. Behind the engine there was a hose completely disconnected. I reconnected it and idle LTFT values got to -1.6% - 0% values - down from earlier 5.5%

2500rpm LTFT values still range form +5.5 to 7.0% so this did not solve all the problems - but some, yes.

What is this hose?
Also, right below the hose + connector (marked by red circle) is two bolt holes / locations. Nothing attached to them. I was curious what are they for. (Bigger on visible in the photo).

Photo of the hose and its location:


I try to document my hunt just in case someone else is struggling with same issues.
Did you ever find out what the disconnected hose is for. My hose is disconnected on the other end and I'm trying to figure out what it goes to.
 
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