My guy locally here has his 93 GT at a shop which will remain private since bashing a particular shop is not my goal. I will first list the cars issues then what the diagnosis has been in each scenario to help inform me if this car should just be pulled from the shop or if the end result is repair.

This whole thing will be a 2 month ordeal the 8th of October.

First: His GT has cams, head, intake, throttle body, completely built basically and nitrous installed but not active

Trick flow track heat upper & lower
AFR 185cc heads
Isky race cam
70mm throttle body
LT headers
30lb injectors

He took the car to a tuner because it needed dyno tuned

*the car was idling rough at low temps. Once warmed up it fine. When the car is cold it kicks up to 1500rpm at start and then drops to 750 almost immediately and stays there until up to temp and then automatically jumps to 1500rpm and runs fine

*the brake pedal was really weak. Almost went to floorpan to engage brake
*needed tune for performance mods and nitrous

Heres the repair parts:

The car was chipped (SCT) and dynoed
Throttle position sensor
new idle air control
4" lighting mass air flow meter
New plugs

Diagnosis:he was told while pulling the car around the idle was due to a vacuum leak which also lent to the weak pedal (only time the pedal was even an issue) so thats only reason I mentioned that. still runs rough no leak found

then it was a throttle position sensor which got replaced. still runs rough
then it was a idle air control. still runs rough
4" lighting mass air flow. still runs rough
new plugs. well much of the same


This is the most recent diagnosis but we dont know what the hell it means "Throttle runs perfect in open loop but as soon as switched to closed loop it runs like crap"

he only wants the car back but it doesnt make any progress at all for some reason. Do you guys have any suggestions?

 

a car with a good sized cam (no idea what he has) at 700-750rpms is gonna have **** for vaccum hece the brakes sucking.

 

ok, that makes sense.

what do you think causes the idle issue? It is directly related to engine temp....dunno?

it just feels like one of those scenarios where the shop might be "guessing" and charging him for every second when the fix is just simple. We both feel like this is being drug out for no reason

 

Well the large cam like i said with such a low idle will deffinatly have problems giving enough vaccum for the brakes. So once the idles kicked up a bit that should be fine. Unless the cam is really big then he would need a vaccum canister. I assume they tuned the car for 30lb injectors so that would help the idleing issues aswell. There could be leaks intake wise but you would have to check them.

But it does sound like the shop is just trying to figure out the problem by throwing his money at it.

 

If they cant find the problem, they can only work from the ground up and they went with common problems which is IAC, TPS, new MAF sensor which seems perfectly normal in my eyes. But like spike said, bigger cams are a common problem for idles like that. I dont know the specs of the cam though.

 

If it's not throwing a code, then all they can do is guess (hopefully educated guess) they went down the list of things that feasibly cause the problem and didn't get to it yet. The garage isn;t going to take a loss on the parts and time spent but if they're serious about Cust Service, then they might give him a break (parts at cost or cut the labor cost a bit) so they don't make a crazy profit. You can leave it there and hope they find it (since they've eliminated many of the culprits) or take it somewhere else and tell them what has been done so far and give another shop a crack at it, but they'll want to charge too.

However, I would think tech's would test the parts (TPS, IAC, MAF etc) to make sure they are operating within the correct voltage ranges before they simply replaced them. Replacing them is something I would do and has proven to be a waste of money at times.

 

well said. THX!!

it just gets frustrating sometimes. when you work on your own ride you know exactly what steps have been done but when other shops work on your stuff and give you just blanket answers without details you just wonder if they really have your best interest at heart or if there just trying to rack up a bill. The trust factor evaporates fairly quickly. Ya know?

 

true dat. I took my little 5.0 to a garage to figure out a rough idle (wasn't sure if it was the cam or a problem). They spent an hour trying different things and called, said they couldn't figure it out and didn;t charge me a dime. I took every problem I ever had to them from that point on. I trusted them.