Trying to Figure out an MPG Problem

CincinnatiFiero · Post by **CincinnatiFiero** » Sat Jan 19, 2008 11:58 am

Alright this isn't my car, its a friends. When I got my hands on his car it was idling at 2,000rpms+ and getting 12 mpg. I put in:

A new Stant 195* Thermostat
New Purolator Fuel Filter
New Purolator Air Filter
Blocked off the EGR Crossover (it was cracked, hence the idle) blocked off the necessary vac lines
Cleaned the IAC off with some pb blaster

She idled at a nice 900rpms and was purring. His passanger side front brake caliper was dragging I guess because the car pulled right and that wheel was covered in brake dust. He had a new line installed, new caliper, and new pads up front. No more pull, no more dust. Also he put in a test pipe in case the stocker was clogged, put in a new set of NGK UR5s, and seafoamed the motor.

He calls me yesterday, 3/4 of a tank got him 80 miles, so his mpg is getting worse. His idle is still good though, but he is running really rich.

Before he bought the car they put in a new O2 sensor and he has the receipt.

So what is it??????
Someone said the IAC could be bad? or the coolant temp sensor? Old Europe suggested dumping injector cleaner in it, which he did, and that didn't help.....
Any ideas?

CincinnatiFiero · Post by **CincinnatiFiero** » Sat Jan 19, 2008 11:59 am

Also he had some part of his clutch system replaced slave cylinder? one of those and the car is driveable but it still needs bled? He said its hard to push, would a not totally functional clutch be boning him?

derangedsheep · Post by **derangedsheep** » Sat Jan 19, 2008 12:45 pm

I would look into replacing the coolant temp sensor. I picked up maybe 2 or 3 MPG when I replaced mine. Its worth a shot, theyre cheap enough.

Xanth · Post by **Xanth** » Sat Jan 19, 2008 6:02 pm

Running rich and getting worse, O2 didn't help? Has he checked to see if the injectors are leaking?

CincinnatiFiero · Post by **CincinnatiFiero** » Sat Jan 19, 2008 6:17 pm

He bought the car used, but the previous owner gave him a receipt saying he had the O2 replaced recently. I don't remember seeing leaky injectors when I pulled the upper plenum. I called him and told him to get a coolant sensor. Hes an hour away from me, I am going to try to get up there in the near future to check out that car, but I think its 2 weeks off at the shortest. Hes a student and 100 miles per tank its getting un-affordable. I may be buying a Silver 87GT soon lol.

he also said he thinks his exhuast manifold is cracked, but it was cracked when I fixed the 2000idle and it was still cracked when the brake got fixed. Yet his car is getting worse. Would a cracked manifold mess him up that much?

Xanth · Post by **Xanth** » Sun Jan 20, 2008 3:18 pm

Perhaps it could badly throw off the O2 reading?

Do you have an ALDL cable you could hook up so you can see what he's dealing with?

CincinnatiFiero · Post by **CincinnatiFiero** » Sun Jan 20, 2008 5:46 pm

No unfortunately I don't have an ALDL cable. I have a friend who has it and he is giving me a quote to replace the leaky manifold so if he replaces the manifold I'll give her a scan.

whipped · Post by **whipped** » Sun Jan 20, 2008 10:00 pm

Leaking cold start injector for the win!

The Dark Side of Will · Mon Jan 21, 2008 11:00 am

Leaking exhaust manifold can make it run rich (maybe not as rich as you describe, though). Air actually gets pulled in through the leak between exhaust pulses, making the O2 sensor think the engine's running lean. The ECM richens the mixture to compensate.

CincinnatiFiero · Post by **CincinnatiFiero** » Mon Jan 21, 2008 6:30 pm

The Dark Side of Will wrote:Leaking exhaust manifold can make it run rich (maybe not as rich as you describe, though). Air actually gets pulled in through the leak between exhaust pulses, making the O2 sensor think the engine's running lean. The ECM richens the mixture to compensate.

That is my current theory as well, is it possible the crack is growing?

The Dark Side of Will · Mon Jan 21, 2008 9:45 pm

All cracks grow. It's the only thing a crack can do.

CincinnatiFiero · Post by **CincinnatiFiero** » Mon Jan 21, 2008 9:55 pm

Well I got a new manifold for him and another buddy is going to put it in this weekend. (I can't do a rear manifold).

The Dark Side of Will · Mon Jan 21, 2008 9:57 pm

CincinnatiFiero wrote:
The Dark Side of Will wrote:Leaking exhaust manifold can make it run rich (maybe not as rich as you describe, though). Air actually gets pulled in through the leak between exhaust pulses, making the O2 sensor think the engine's running lean. The ECM richens the mixture to compensate.
That is my current theory as well, is it possible the crack is growing?

Another symptom of this is that it should also pop loudly and maybe blow out your muffler coasting down from high RPM.

CincinnatiFiero · Post by **CincinnatiFiero** » Tue Jan 22, 2008 4:44 pm

Well hopefully this weekend I am getting a buddy to install a new header for him. Hopefully it fixes it, I told him to buy a new coolant sensor anyways and to double check on any fuel leaks.

p8ntman442 · Post by **p8ntman442** » Tue Jan 22, 2008 6:22 pm

Stuck cold start injector, or hole in the gas tank.

CincinnatiFiero · Post by **CincinnatiFiero** » Wed Jan 23, 2008 5:01 pm

Hole in the gas tank is kind of obvious, its not that, and he said gas isn't leaking anywhere. I am pretty much positive at this point that its the header.

Fastback86 · Post by **Fastback86** » Wed Jan 23, 2008 7:08 pm

CincinnatiFiero wrote:Hole in the gas tank is kind of obvious, its not that, and he said gas isn't leaking anywhere. I am pretty much positive at this point that its the header.

A "leaky" Cold Start Injector is one that is stuck open and leaking fuel into the lower intake manifold and thus right into the cylinders and making your car run rich. If the manifold doesn't fix the problem, I'd check the Cold Start Injector next.

Kohburn · Post by **Kohburn** » Thu Jan 24, 2008 9:43 am

the cold start injector switch is also a possability. you can just unplug the sensor/switch and see if that fixes it before replacing the injector its self.

I_wear_pants · Post by **I_wear_pants** » Wed Jan 30, 2008 9:16 pm

I think a coolant temperature sensor is a possibility if the manifold doesnt fix it. The sensor could be reading maximun cold all of the time and the ecm sending the signal to run rich all of the time as a result. If that is the case I think that could cause the cold start injector to run constantly since it never sees the coolant warming up...

Series8217 · Post by **Series8217** » Wed Jan 30, 2008 10:03 pm

The ECM doesn't control the cold start injector, does it? Isn't there a seperate coolant temp switch for the cold start injector?