Hi, I live in the UK so I don’t have many garages I can take my 99 auto Z28 (LS1) to to be repaired so I am hoping someone here may be able to help.
The problem started a few months ago, where the idle speed goes up as soon as the car starts to move, sounds odd but please keep reading.
If I stop on a hill with the break on and the car in neutral, then I let the car roll forward, still in neutral then the idle speed goes up, sometime as far as 1500 RPM, the other thing I have noticed is when driving at normal speed (60mph), if I take my foot off the gas the engine is does not want to slow the car down.
Soon after this started I asked my local garage and he said I should use some good quality (Wynns) fuel injector cleaned, about a month after that the engine warning came on, the car seemed fine and a couple of days later it went off and it has been going on and off every 2 or 3 days ever since.
A month after this all started it went in for a service so oil and filters were all replaced
Fuel consumption has gone from 22mpg to about 17.

 

The idle will speed up with the speed of the car. Mine does it.

The problem with the engine not wanting to slow down, I've had. Try adjusting your throttle cable. Mine was adjusted too tight and my RPM would increase when I pushed in the clutch. There is a plastic junction in the engine bay that you can use to adjust it. If you have cruise control there are to places to adjust it. Just follow the cale from your throttle body and you'll find it. Open it up and slide it for adjustment and lcose it when you have it where you want. I don't guarantee this is the problem though.

For your engine light, you really need the code read. Is it a "check engine" light or a "service engine soon" light? If I had to guess w/the fuel consumption and seeing that you changed your filters, I'd say its an O2 sensor problem.

 

I would guess that your IAC (idle air controller) is getting bad or is just plain dirty. Another suspect in this would be the throttle position sensor.
Not having good nearby reliable garages to help you out you really need to have a good scan tool so that you can see what all of the sensors are reporting to the PCM.

 

Thanks for the quick replies,
I don't think it's a sticky throttle cable because the revs go up as soon as the car starts rolling forward and then slows down just after it stops moving, it's like there is a speed motion sensor that ups the revs all the time the car is moving.

The Owners manual says the warning light is called "Malfunction Indicatar", it also says if it stays on to make sure the filler cap is properly installed.

I have removed and cleaned the IAC but it looked OK anyway.

Sounds like it's going to be an expensive trip the garage, $100 just to connect the scan tool, then a few more $100 while they blindly swop bits.

One thing I did notice was the air filter box was loose and there was some loose dirt stuck on the inlet the the engine, I assume thats the MAF, anyway I cleaned it all out and changed the air filter again but it's still the same.

 

unplug the VSS from the trans and see if it still idles up at a stop...also you should be able to order a hand held scaner for your car for around $100....then you never have to take the car in to get it scaned

 

Sparkz28ss, thanks for the help – If I unplug the engine vss then the ecu won’t know how fast the engine is going so I guess the idle speed will be uncontrolled and the rev counter/speedo won’t work either, what will happen if I try to drive it like this, will it be safe? And where on the trans (it’s an auto) is the connector?

 

If your book says the light relates to the filler cap, then that is the service engine soon light. The SES light relates to emissions equipment. If you can't get codes read, then yes make sure your cap is on tight. It can take a few start up cycles to turn off or you can reset the ECU.

I agree w/the buying a scanner idea if they're going to charge you to read codes. Here we can got to part stores and get the codes read for us.

Honestly its all just a guessing game until you know what codes are being thrown.

 

OK, I just got back from the garage and the scanner came up with the following codes saved in history:
P0171 Fuel System too Lean (Bank 1)
P0174 Fuel System too Lean (Bank 2)
P0507 Idle Control System RPM too high
All other codes were clear, no misfires and we even adjusted the rpms so see if the air speed control was sticking
We cleared the history and the engine (SES) warning light but by the time I got home 8 miles and 15 minutes later the light was back on.
The garage said the car was OK and not to worry, it could be an O2 sensor but he is not sure.
What he did not want to do was charge me for randomly replacing parts which would become very expensive.
My concern is if the engine is running “too lean” then that could cause major problems.
What do you recon?

 

Did you check your fuel pressure. Double lean could be a bad fuel pump.

 

I have to rely on my garage in the UK for tests or checks because I don't have my own scanner.
One thing I said ealier was: ###The air filter box was loose and there was some loose dirt stuck on the inlet the the engine, I assume thats the MAF, anyway I cleaned it all out and changed the air filter again but it's still the same.###

Could this be a clue, could there be an air leak where the air filter is, if so would this cause the error codes?
Another thing, but this may be a red herring, there is a "hissing" thye of noise, the garage said this was from the pullies, I see from a sticky this can be quite common

 

There are many possible causes for lean codes on both banks, but the 2 most common are contaminated MAF, and a vacuum leak sourcing from your PCV tubing. You need to thoroughly inspected the PCV system to make sure none of the hoses have deteriorated to the point of creating a leak.

 

Thanks Marc 85Z28,
I did not remove it but as far as I can see the PCV tubing looks fine, so the next thing is the MAF, if it was dirty or contaminated then I thought I would be getting an error to say something like Fuel System too Rich, not too lean.
Is it worth cleaning with MAF or Carb spray?

 

Is it worth cleaning? Absolutely - it's free!

And a contaminated MAF can throw either rich or lean codes. More often than not, I see contaminated MAFs setting lean codes - this goes for every vehicle manufacturer I've ever dealt with. With a little know how and a scanner that can datalog, a contaminated MAF is an easy diagnosis. They'll often skew their measurements. They will read rich at idle, close to correct at part throttle, and will read lean with higher throttle inputs.

If cleaning the MAF doesn't fix the problem, you can spray the same can of MAF/carb cleaner near all intake connections and where the intake port seals contact the intake and head. If the idle fluctuates, you have found a leak.

 

Marc 85Z28, thanks again.
Great explanation, I have been looking all afternoon but I can't get MAF cleaner in the UK, nearest thing I can find is either "Wynns Fuel Injection Intake & Carb Cleaner" or I have some non-flammable electrical cleaning solvent.
I read on another forum the the guy just sprayed it in short bursts while the engine was running, is this bad advice, or should I carefully remove the MAF and do it that way?

 

use the electrical cleaner, not the carb spray.... pull it off the car and put it on a workbench and give it a few blasts with the cleaner...you may need a cotton swab to clean the junk off of the thermistors,but be very careful and dont leave any of the cotton bits on the thermistors