: 1993 Northstar Rough Idle

04-18-08, 05:33 PM
I have a 93 Allante with a Northstar engine with 63k on the odometer. It idles rough and the engine often dies. When this happens and I restart, it doesn't want to start very easily. If I put the car in neutral, it idles smoother and does not die. The problem started when I stored the car for 7 months. Also, the fuel economy is terrible on this car especially around town. I get 10 mpg driving like there is an egg on the accelerator pedal around town. With the same driving situation, my 88 Eldorado will bring is around 20 mpg.

I have done the following in this order:
- Replaced bad coil with Autozone coil. This improved the power but did not improve fuel economy. It did also help the idle somewhat.
- Ran Lucas fuel treatment. It did not make any difference.
- Used 91 octane fuel instead of 87 octane. It made no difference.
- Cleaned EGR mouse holes. No difference
- Cleaned Throttle Body blades. No difference
- Cleaned EGR Valve. No difference
- Installed new MSD 8.5mm Spark Plug wires. Idle did improve but it still stalls when the transmission is in gear. It also eliminated the engine stutter that would occur at 55 mph going up a hill.
- Regapped original AC Delco spark plugs to .040 All the plugs looked clean and not worn and they were all at .055 It made maybe a slight difference.
- Changed fuel filter. No difference

Here are some things maybe I should look at.
1. MAP sensor
2. FPR
3. TPS
4. Fuel Injectors

Where should I start?

04-19-08, 08:21 PM
Where should I start?

04-20-08, 11:44 AM

A mechanic told me that I could test if the FPR was leaking by turning on the key (when engine is hot but not starting the engine) and letting it sit for a couple of minutes then turn off the key, and keep doing this several times. After about 10 minutes of doing this, I went to restart the engine and it started without a problem. The mechanic said if the FPR is bad, the engine would be flooded with fuel and would probably not want to start.

Is this a valid test?

04-20-08, 02:34 PM
Not really Ed. Turning the key on will only run the fuel pump for 2-3 seconds. Then if not cranked it will shut down. Remove the manifold cover and do it again. Once the fuel pump runs and pressurizes the system, look for fuel leaking from the FPR nipple. If it is leaking, replace it. Doing it his way would take an awful lot of fuel to fill the manifold and flood the cylinders. That could take forever. If it is not leaking, I'd get a fuel pressure gage on the service port and check your fuel pressure both static and again while cranking. Also at idle if you get it started.