84 Crossfire Injection
Thread Starter
Racer
Joined: Oct 2010
Posts: 403
Likes: 5
From: West Palm Beach FL
84 Crossfire Injection
HI all....I am new here and jsut got my 1st vette a couple of months ago. It ran rough so i got it a tune-up last week.....It runs MUCH better but it has a stumble at idle and sometimes idles a bit rough. Also, when its hot, its sometimes hard to start, acting like the choke is stuck or its flooded. I know there is no choke. Any ideas?
Burning Brakes
Joined: Jun 2008
Posts: 895
Likes: 3
From: toledo ohio
i dont have an 84 but i do have an 82 and well, same engine. i had a similar problem with the hard to start and had i guess a "searching" idle it would rev up and down between 500 rpm to 1500 rpm, got a lot of tips on vacuum leak egr etc but in the end it was a failing fuel pump. might wanna check your fuel pump and fuel filter, pretty easy to access on the c4 (well fuel pump anyways) and arent too expensive. a delco one is like 70 i think or you can get the racetronix/walboro package (which i recommend, no wire splicing) for a little over a hundred. even if its not you ultimate problem its something good to replace on a car that has sat for a while. i know ive been stranded by a fuel pump failing. good luck tracking it down!
Le Mans Master
Joined: May 2006
Posts: 9,226
Likes: 9
From: Irvine Ca
Drifting
Joined: Jan 2006
Posts: 1,372
Likes: 2
From: Ramona CA
I had an '84 and it ran strong but not until I did a few things.
1. '84 CFI sounds real rough when starting cold. The air intakes in the hood connect to the air filter housing. One of the air intakes keeps this 'flap' closed when the engine is cold causing a very rough idle. I believe its spring loaded 'open' with a vacuum actuator keeping it close on cold start. I think I removed the vacuum line to it causing both air flaps to stay open. This eliminated rough idle at cold startup.
2. Get '85+ fuel pump. '84 FP is a weak sister. The '85 FP will eliminate hesitation providing fuel filter is clean and injectors are OK.
3. Add Ram air intake. Cut a hole in the air box with a moto drill the size of the air intake in the hood. Use good house hold weather stripping around the hole for sealing. Helps breathing with cold air at WOT. You'll see in one of my engine pictures what I'm talking about.
http://forums.corvetteforum.com/c4-t...king-84-a.html
4. Get a Renegade. Much improved breathing. Increase your red line beyond 4800 to about 5800.
5. Get 2" TBs. Won't fuel starve at high rpm.
6. Get higher compression heads.
It'll run like a bat out of hell and go kick some C5 automatics butt.
1. '84 CFI sounds real rough when starting cold. The air intakes in the hood connect to the air filter housing. One of the air intakes keeps this 'flap' closed when the engine is cold causing a very rough idle. I believe its spring loaded 'open' with a vacuum actuator keeping it close on cold start. I think I removed the vacuum line to it causing both air flaps to stay open. This eliminated rough idle at cold startup.
2. Get '85+ fuel pump. '84 FP is a weak sister. The '85 FP will eliminate hesitation providing fuel filter is clean and injectors are OK.
3. Add Ram air intake. Cut a hole in the air box with a moto drill the size of the air intake in the hood. Use good house hold weather stripping around the hole for sealing. Helps breathing with cold air at WOT. You'll see in one of my engine pictures what I'm talking about.
http://forums.corvetteforum.com/c4-t...king-84-a.html
4. Get a Renegade. Much improved breathing. Increase your red line beyond 4800 to about 5800.
5. Get 2" TBs. Won't fuel starve at high rpm.
6. Get higher compression heads.
It'll run like a bat out of hell and go kick some C5 automatics butt.
Melting Slicks
Joined: May 2009
Posts: 2,772
Likes: 175
From: Decatur IL
2025 C4 of the Year Finalist - Unmodified
2022 C4 of the Year Finalist - Unmodified
Sounds like a fuel pump issue. Chances are it could be yhe original one or one that was replaced a long while ago' I had an 84 that had similar issues and the pump finally died. I replaced it with an 85 pump and solved all my issues. Hope this helps.
Drifting
Joined: Nov 2009
Posts: 1,341
Likes: 2
From: Perth Western Australia
St. Jude Donor '10
Add Ram air intake. Cut a hole in the air box with a moto drill the size of the air intake in the hood. Use good house hold weather stripping around the hole for sealing. Helps breathing with cold air at WOT. You'll see in one of my engine pictures what I'm talking about.
http://forums.corvetteforum.com/c4-t...king-84-a.html
http://forums.corvetteforum.com/c4-t...king-84-a.html
Melting Slicks
Joined: Dec 2009
Posts: 2,231
Likes: 284
Check the coolant temp sensor, its located in the front of the intake near the air pump. The 84s had an early style sensor that can be troublesome, get a later style sensor & pigtail connector.
You can try wiggling the sensor to see if it acts up.
You can try wiggling the sensor to see if it acts up.
Drifting
Joined: Jan 2006
Posts: 1,372
Likes: 2
From: Ramona CA
In the picture, in front of the fan there is the radiator housing. Directly front of that is the air box. Notice the square hole in the air box. Also, notice the the hood meets the air box and the hole in the hood used to draw air. When the hood closes the the hole in the hood is directly over the hole in the air box drawing cold air. Normally '84s don't have that hole in the air box cutting off the air when the hood is closed. Really noticeable at wide open throttle (WOT).
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
Drifting
Joined: Nov 2009
Posts: 1,341
Likes: 2
From: Perth Western Australia
St. Jude Donor '10
In the picture, in front of the fan there is the radiator housing. Directly front of that is the air box. Notice the square hole in the air box. Also, notice the the hood meets the air box and the hole in the hood used to draw air. When the hood closes the the hole in the hood is directly over the hole in the air box drawing cold air. Normally '84s don't have that hole in the air box cutting off the air when the hood is closed. Really noticeable at wide open throttle (WOT).
Le Mans Master
Joined: Nov 2009
Posts: 6,882
Likes: 234
From: Bangkok, Thailand
Why oh why do OEM's do so many things like that!?!?!? I would bet that the increased intake noise from opening that up was a problem, so they left it for us to do if we wanted to. Noise standards can be a funky thing.
To the OP, I'd take a look at a worn throttle position sensor. It could cause this sort of problem too.
Last edited by H P Bushrod; Oct 28, 2010 at 10:10 AM. Reason: nsv link removed
Safety Car
Joined: Nov 2005
Posts: 3,532
Likes: 1,185
From: Arizona - If you don’t know CFI, STOP proliferating the myths around it...
Check your fuel pressure in-between the TBs. CFI is rather sensitive to fuel pressure. Set it for at least 13-14psi and that should be fine. Next, what is the TPS set to? Set it at .525v or as close to that as possible. Set it with...Key ON, engine OFF. If these two don't help, start looking for vacuum leaks, check that top plate. Good luck.
Drifting
Joined: Jan 2006
Posts: 1,372
Likes: 2
From: Ramona CA
Rumor has it in '83 GM had the TPI working with much better performance than the CFI. The rumor is they purposely did things to reduce CFI performance until TPI was production ready. When you take the CFI intake manifold off you'll see the intake runners are half the size on the manifold side than the head side. They plugged it up on purpose. Maybe TPI was less expensive in production. Many times the automotive engineers were told what to do by management to save a dime.
Bottom line - make a CFI breath and not starved for fuel or air and it gets impressive. Renegade has replaced X-Ram. With a Renegade intake manifold and 2" TBs and some of the tips I mentioned your L83 CFI will rock. The guy who was selling X-Ram was getting about 450 hp from an '84 CFI with a stroked 383.
Bottom line - make a CFI breath and not starved for fuel or air and it gets impressive. Renegade has replaced X-Ram. With a Renegade intake manifold and 2" TBs and some of the tips I mentioned your L83 CFI will rock. The guy who was selling X-Ram was getting about 450 hp from an '84 CFI with a stroked 383.
Team Owner
Joined: Aug 2005
Posts: 34,988
Likes: 515
From: Riverside County Southern California
Co-winner 2020 C4 of the Year - Modified
2018 Corvette of Year Finalist
2017 C4 of Year
2016 C7 of Year Finalist
St. Jude Donor '09 thru '20
Rumor has it in '83 GM had the TPI working with much better performance than the CFI. The rumor is they purposely did things to reduce CFI performance until TPI was production ready. When you take the CFI intake manifold off you'll see the intake runners are half the size on the manifold side than the head side. They plugged it up on purpose. Maybe TPI was less expensive in production. Many times the automotive engineers were told what to do by management to save a dime.
Bottom line - make a CFI breath and not starved for fuel or air and it gets impressive. Renegade has replaced X-Ram. With a Renegade intake manifold and 2" TBs and some of the tips I mentioned your L83 CFI will rock. The guy who was selling X-Ram was getting about 450 hp from an '84 CFI with a stroked 383.
Bottom line - make a CFI breath and not starved for fuel or air and it gets impressive. Renegade has replaced X-Ram. With a Renegade intake manifold and 2" TBs and some of the tips I mentioned your L83 CFI will rock. The guy who was selling X-Ram was getting about 450 hp from an '84 CFI with a stroked 383.
Thread Starter
Racer
Joined: Oct 2010
Posts: 403
Likes: 5
From: West Palm Beach FL
Drifting
Joined: Nov 2009
Posts: 1,341
Likes: 2
From: Perth Western Australia
St. Jude Donor '10
If all you have is a drill then get a small bit and drill a hundred little holes around the perimeter and then remove the rectangle cut out and then smooth with a file. That is the way I did it.
Last edited by Lemme; Oct 29, 2010 at 07:27 PM.
Thread Starter
Racer
Joined: Oct 2010
Posts: 403
Likes: 5
From: West Palm Beach FL
I just did it by eye. Otherwise get some cardboard and cut out a template, draw around the cardboard on the top of the airbox with a marker and then drill holes in the corners. Then get out your jigsaw / hacksaw and cut. It is fibreglass so it cuts easily. Don't breathe the dust. The weatherstrip is 25mm wide by 19mm deep. Total cost less than $5.
If all you have is a drill then get a small bit and drill a hundred little holes around the perimeter and then remove the rectangle cut out and then smooth with a file. That is the way I did it.
If all you have is a drill then get a small bit and drill a hundred little holes around the perimeter and then remove the rectangle cut out and then smooth with a file. That is the way I did it.
Drifting
Joined: Nov 2009
Posts: 1,341
Likes: 2
From: Perth Western Australia
St. Jude Donor '10
No regrets. When my new renegade intake manifold arrives I want to let as much air through it as possible.
Le Mans Master
Joined: Nov 2009
Posts: 6,882
Likes: 234
From: Bangkok, Thailand
