running rough
Thread Starter
6th Gear
Joined: Mar 2017
Posts: 6
Likes: 0
running rough
ok guys, just bought a 91 with 150k miles. ran like crap when i got it . ( bought as a project car) auto trans .
i got home and did the basic...plugs,wires, cap, rotor, pcv valve, fuel filter, made sure the fuel pump works, and replaced the coolant temp sensor..
it runs better, still rough, i put my finger on the injectors while it idled and only half i could feel were vibrating, 2 on each side. no i havent checked the ohms, was gonna do that tomorrow.
also the only way it will start, is if i have the pedal all the way to the floor and it cranks a few times. it wont start otherwise.
im thinking its the fuel injectors, as 5 look original with purple where the wires go in,2 were red and one was purple with a green line at the base...
also my tach doesnt work when the car is running. once it fires with my foot on the pedal it jumps to 9k then drops back to zero.
please help!!!!!
i got home and did the basic...plugs,wires, cap, rotor, pcv valve, fuel filter, made sure the fuel pump works, and replaced the coolant temp sensor..
it runs better, still rough, i put my finger on the injectors while it idled and only half i could feel were vibrating, 2 on each side. no i havent checked the ohms, was gonna do that tomorrow.
also the only way it will start, is if i have the pedal all the way to the floor and it cranks a few times. it wont start otherwise.
im thinking its the fuel injectors, as 5 look original with purple where the wires go in,2 were red and one was purple with a green line at the base...
also my tach doesnt work when the car is running. once it fires with my foot on the pedal it jumps to 9k then drops back to zero.
please help!!!!!
Drifting
Joined: Mar 2013
Posts: 1,952
Likes: 239
From: Venice Cali.
Last edited by xrav22; Mar 24, 2017 at 10:22 PM.
Flatulent Dwarf
Joined: Nov 2009
Posts: 729
Likes: 84
From: Poverty Holler NC
I don't have a lot of experience with that engine so I have no "sage" advice to offer, but having to fully depress the accelerator just to crank it ...kinda sounds like a sensor or metering device problem, or a hacked up harness. A lot of Corvette problems come from crappy batteries or terminals loose. 
later, tiny
later, tiny
Race Director
Joined: Feb 2003
Posts: 16,475
Likes: 270
From: Kingman AZ
Cruise-In VII Veteran
Cruise-In VIII
St. Jude Donor '06-'07-'08-'09-'10-'11-'12-'13-'14-'15-'16
Mice love to chew on the injector wires (ask how I know). Looks like they were replaced at one time.
Do a Google search for 91 Corvette tachometer fix. The 90 and 91s were notorious for this.
See if you can rent or borrow a fuel pressure gauge. It will connect right at the fuel rail.
Clean the throttle body with spray cleaner.
Good luck!
Do a Google search for 91 Corvette tachometer fix. The 90 and 91s were notorious for this.
See if you can rent or borrow a fuel pressure gauge. It will connect right at the fuel rail.
Clean the throttle body with spray cleaner.
Good luck!
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Le Mans Master
Joined: May 2006
Posts: 7,708
Likes: 266
From: indy indiana
sounds like you are starting with zero fuel pressure when cold cranking. Turn the key on, listen for the fuel pump to kick off. Turn it to start for maybe two or three revolutions. Do the three times, bet it starts the fourth Without giving it any fuel at all. Check for vacuum leaks on the rough running at idle. Good place to start.
Thread Starter
6th Gear
Joined: Mar 2017
Posts: 6
Likes: 0
moved the car last night( so i can get my other project out of the garage)
went to autozone and got both oil pressure sensors, replaced them, got in it and it fired right up, turned it off, waited like 10 minutes and it wouldnt fire... bought the noid light test set.. all fuel injectors are getting proper signal.. got it running, and unplugged each injector.. some seemed like no change and some there was a change when unplugging, so some of my injectors are shot.. would explain the roughness, but would that cure the no start, as its not firing on all cylinders? also im seeing alot of fuel onthe air side of the throttle body and in the tube to the air filter.
went to autozone and got both oil pressure sensors, replaced them, got in it and it fired right up, turned it off, waited like 10 minutes and it wouldnt fire... bought the noid light test set.. all fuel injectors are getting proper signal.. got it running, and unplugged each injector.. some seemed like no change and some there was a change when unplugging, so some of my injectors are shot.. would explain the roughness, but would that cure the no start, as its not firing on all cylinders? also im seeing alot of fuel onthe air side of the throttle body and in the tube to the air filter.
Burning Brakes
Joined: Jul 2013
Posts: 1,027
Likes: 30
From: Granite City Illinois
Maybe it will if theyre leaking or the o rings are leaking. My 84 had leaking o rings and it wouldn't start easily once the engine was warmed I had to hold the gas pedal down to get it started and lots of black smoke poor mpg etc
check the fuel pressure regulator line for fuel smell it could be unsafe if its bad. Might smell the oil on the dipstick for fuel odor
check the fuel pressure regulator line for fuel smell it could be unsafe if its bad. Might smell the oil on the dipstick for fuel odor
Last edited by ToniJ1960; Mar 25, 2017 at 01:54 PM.
Drifting
Joined: Mar 2011
Posts: 1,693
Likes: 144
From: on Barnegat Bay Joisey
With 3 different looking injectors I would replace the set. Call Jon at fic.
My experience is that once the multechs start to go they will all go eventually. Mine started missing/nostart 3 years ago and goodwrench replaced 2, cost about $800. The injectors were $150 each over the counter. Three months later the trouble started again so I bought a complete set from Jon and did the job myself. Took me about 6 hours. Changing them piecemeal is just a waste of time.
Once you have the top of the motor off its a good idea to replace all the vacuum lines which are accessible.
My experience is that once the multechs start to go they will all go eventually. Mine started missing/nostart 3 years ago and goodwrench replaced 2, cost about $800. The injectors were $150 each over the counter. Three months later the trouble started again so I bought a complete set from Jon and did the job myself. Took me about 6 hours. Changing them piecemeal is just a waste of time.
Once you have the top of the motor off its a good idea to replace all the vacuum lines which are accessible.
Thread Starter
6th Gear
Joined: Mar 2017
Posts: 6
Likes: 0
Yea I went ahead and ordered a flow matched set, fuel regulator, and gasket kit from him over the weekend. And yea good idea on the vacuum lines. While in in there I'm gonna do valve cover gaskets as well. Was also thinking about getting the valve covers and intake dip cleaned too, depending how bad they look