I have a '99 STS with a strange issue; when starting the car from cold it fires up first time no problem without having to press gas pedal. Once car has been used it takes several cranks and often requires the gas pedal pressed to start, but here's the thing, when cranking it will (appears to) backfire through the intake manifold.

While driving sometimes there appears to be flat spots and car will not kick down when pressing gas pedal firmly, but other times it drives great.

This car has no DTC's and is babied by me mechanically and I only use genuine GM parts. Here is a list of all new items replaced trying to sort this issue:

Intake manifold gaskets
Crank position sensor
Spark plugs
Spark leads
Coil pack
MAF
Air filter
Fuel filter

I run the car on either Shell V-Power or BP Premium fuels. I did book the car in for a fuel pressure test last week but the garage I take it to did not have the correct fuel line adaptors to connect the pressure test kit. This will be done latter this month.

Any of you technicians/mechanics out there have any ideas?

Regards n thanks.

 

clean the throttle body check iac
also u need to find out if u have a leaking injector

strange its a standard fuel pressure gauge hook up
by them at any car store

 

Sorry, I meant to say that the throttle body and intake system has been thoroughly cleaned/de-carboned.

I am in the UK so possibly what is 'standard' in the USA is not here.

 

I see you never mentioned temp. sensor or MAP sensor.

 

No,...and your point is? lol

There is no indication that either sensor is faulty but if you want to provide some additional explanation as to which angle you are coming from that could be most helpful

 

if u have a spare iac i would replace that
did u clean it
does the vehicle start easy if u put the pedal to the floor, this is fuel cutoff mode
or does it start better with pedal a little bit down
u really need a scanner to pin point it but u need to see the throttle position sensor and the iac position

 

Thanks GMTech, it's been on the diagnostics and there are no codes!

I will check the IAC and TPS though. Finally going to get the fuel system pressure tested Tuesday.

 

Update:

Fuel pressure test revealed a constant pressure of 45psi with no apparent drop.

Removed TPS AND IAC, throttle linkage moves freely with no sticking. IAC had heavy carbon deposit so cleaned that but the springloaded probe seems very stiff and does not move easily, is this normal?

I have ordered a new TPS and IAC anyway and will fit those when they come but in the interim will see what effect cleaning the IAC has made.

...more to follow no doubt

 

its is usually a little stiff, did u clean out the hole the iac was in and the throttle plate

 

Oh yes I used carb/injector cleaner spray and a little stiff artists brush and thoroughly cleaned everything.

My new sensors have been ordered so just need to wait for them to arrive from across the 'pond'.