84 hesitating bad under throttle. Please Help!
Thread Starter
Intermediate
Joined: Oct 2012
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84 hesitating bad under throttle. Please Help!
84 vette that has been maintained. I got a check engine light a few times last week. I read the code and it was 13-oxygen sensor. The car ran good, but I replaced it since the CEL came on more than once. So, I replaced the 02 sensor with an oem sensor and the car started running terrible.
It idles fine, but under throttle it's hesitating bad. I changed the o2 sensor again, thinking I may have gotten a bad one from the dealer. It's still running rough. It hesitates after you are going about 15 mph. I've tried mashing throttle, medium throttle, and light throttle. It does it under any circumstance. There are no codes present. It had new fuel filter, fuel pump, plugs, wires, cap etc only a year ago. I'm confused because it ran fine before replacing the sensor. Is it possible the cat converter is clogged and it showed up in the system as oxygen sensor? - but why would new 02 sensor make it worse?
It feels like it's running on 4 cylinders and I swear the car is much quieter now- and not it a good way. Motor was completely rebuilt less than three years ago, so I'm thinking it is something simple. Also, put some injection cleaner and iso heet thinking I may have picked up bad gas...didn't help. Thank you for any suggestions.
It idles fine, but under throttle it's hesitating bad. I changed the o2 sensor again, thinking I may have gotten a bad one from the dealer. It's still running rough. It hesitates after you are going about 15 mph. I've tried mashing throttle, medium throttle, and light throttle. It does it under any circumstance. There are no codes present. It had new fuel filter, fuel pump, plugs, wires, cap etc only a year ago. I'm confused because it ran fine before replacing the sensor. Is it possible the cat converter is clogged and it showed up in the system as oxygen sensor? - but why would new 02 sensor make it worse?
It feels like it's running on 4 cylinders and I swear the car is much quieter now- and not it a good way. Motor was completely rebuilt less than three years ago, so I'm thinking it is something simple. Also, put some injection cleaner and iso heet thinking I may have picked up bad gas...didn't help. Thank you for any suggestions.
Team Owner
Joined: Aug 2004
Posts: 21,543
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From: Park City Utah
Step 1 on this car: Check the Fuel pressure.
I know you changed the pump etc a year ago. Doesn't matter (especially if you used an '84 replacement pump).
If you REALLY think it's the cat, install a vacuum gauge. Should read somewhere around 18" at idle. Crack the throttle and hold steady at say, 2500 RPM. What SHOULD happen is the vacuum gauge should drop sharply as you crack the throttle, then rebound quickly as RPM stabilize at your target (~2500) and end up at about 21" or so.
IF your cat (or muffs) is clogged, then what will happen is you'll see the gauge drop, rebound (as would be normal) then start dropping away again. That is as pressure builds in the exhaust, it also builds in the intake too...giving you a lower vacuum reading. That is a dead give-away. Falling off vacuum at high RPM/low load is a sure sign of a backed up exhaust.
I know you changed the pump etc a year ago. Doesn't matter (especially if you used an '84 replacement pump).
If you REALLY think it's the cat, install a vacuum gauge. Should read somewhere around 18" at idle. Crack the throttle and hold steady at say, 2500 RPM. What SHOULD happen is the vacuum gauge should drop sharply as you crack the throttle, then rebound quickly as RPM stabilize at your target (~2500) and end up at about 21" or so.
IF your cat (or muffs) is clogged, then what will happen is you'll see the gauge drop, rebound (as would be normal) then start dropping away again. That is as pressure builds in the exhaust, it also builds in the intake too...giving you a lower vacuum reading. That is a dead give-away. Falling off vacuum at high RPM/low load is a sure sign of a backed up exhaust.
Racer
Joined: Oct 2012
Posts: 294
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From: Spokane WA
Probably is your cat my dodge pickup did that to. Unbolt the cat and rev it up. You can buy a straight pipe off of eBay for $50 bolts right up. Boost in performance and never worry about a plugged cat again.
Thread Starter
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How do you check fuel pressure? Would it be easier to just replace the fuel pump? I read somewhere that it's a pretty easy job. As far as the straight pipe on ebay, consider it done. I'm sure it's a noticeable difference and I'll never have to worry about it again. I appreciate the recommendations guys.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
If you have a gauge, it's easy. Just "T" into the fuel line by the filter using some fuel hose, hose clamps and a "T".
Spec is 9-13, but you're looking for 12-14 PSI.
Replacing the pump is pretty easy. Not as easy as checking pressure, but close. Why waste money though? Diagnose the problem.
Same goes for you "clogged cat". Diagnose first, then replace.
Spec is 9-13, but you're looking for 12-14 PSI.
Replacing the pump is pretty easy. Not as easy as checking pressure, but close. Why waste money though? Diagnose the problem.
Same goes for you "clogged cat". Diagnose first, then replace.
Thread Starter
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I don't have a gauge to check fuel pressure. I'm going to take it to an exhaust shop tomorrow. They said they would test it for free. Already bought new plugs, so I plan to do those after work tonight. May pick up a fuel filter as well. It looks easy to change since the location is easily accessible. Maybe that will take care, but I suspect the cat is the issue. I'll keep you posted and thanks for the replies Tom!
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Thread Starter
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I have good news, and some not so good news. I deleted the cat and ran a straight pipe as someone suggested. It didn't change anything as far as the hestitation goes. I finally got to the plugs this morning and...WOW what a difference. The vette runs better than it ever has! 
One of the plugs was awful. It had grease and oil all over the end of it. The rest looked pretty good, but that one plug definitely was the problem.
Now my concern is what caused that. Could the 02 sensor being bad cause any of it, or is it something worse? Thankfully it's the easiest plug to fix, so I may just replace that one every 6 months or so. Any input on the dirty plug? Thanks again for everyone's input!
One of the plugs was awful. It had grease and oil all over the end of it. The rest looked pretty good, but that one plug definitely was the problem. Now my concern is what caused that. Could the 02 sensor being bad cause any of it, or is it something worse? Thankfully it's the easiest plug to fix, so I may just replace that one every 6 months or so. Any input on the dirty plug? Thanks again for everyone's input!
Team Owner
Joined: Aug 2004
Posts: 21,543
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From: Park City Utah
Ah HAH...so it WASN'T the cat! See why it pays to DIAGNOSE, rather than guess (ahem! 1984crossfirefuelie)
An improper A/F ratio (cause by a Faulty O2) coudl blacken a plug. The soot could build to the point where it was literally "chunks" on the end of the plug, but it would be a dull/flat black...not "oily/greasy". Oil/Grease looking is probably caused by...oil. Run it for a bit, and pull that plug again for inspection. If it's oily looking again, time for a compression test, or a leak-down test. Are you consuming (losing) oil?
An improper A/F ratio (cause by a Faulty O2) coudl blacken a plug. The soot could build to the point where it was literally "chunks" on the end of the plug, but it would be a dull/flat black...not "oily/greasy". Oil/Grease looking is probably caused by...oil. Run it for a bit, and pull that plug again for inspection. If it's oily looking again, time for a compression test, or a leak-down test. Are you consuming (losing) oil?
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That is what I wanted to hear. It isn't oily, its chunky grease like you just described. The straight pipe was only $40 installed and I never have to worry about the cat. When I get home tonight I will upload a pic of the plug, but yes it was def chunky grease. It doesn't use or burn oil, so I'm guessing it was that crappy o2 sensor. Guys, change em out before they give you issues. Best $20 buy imo. Thanks for the reply.
An improper A/F ratio (cause by a Faulty O2) coudl blacken a plug. The soot could build to the point where it was literally "chunks" on the end of the plug, but it would be a dull/flat black...not "oily/greasy". Oil/Grease looking is probably caused by...oil. Run it for a bit, and pull that plug again for inspection. If it's oily looking again, time for a compression test, or a leak-down test. Are you consuming (losing) oil?[/QUOTE]
An improper A/F ratio (cause by a Faulty O2) coudl blacken a plug. The soot could build to the point where it was literally "chunks" on the end of the plug, but it would be a dull/flat black...not "oily/greasy". Oil/Grease looking is probably caused by...oil. Run it for a bit, and pull that plug again for inspection. If it's oily looking again, time for a compression test, or a leak-down test. Are you consuming (losing) oil?[/QUOTE]
Thread Starter
Intermediate
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Here is a pic of the plug. What do you think Tom? Definitely greasy!
http://i113.photobucket.com/albums/n...2/IMAG0520.jpg\
http://i113.photobucket.com/albums/n...2/IMAG0520.jpg\
Racer
Joined: Oct 2012
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From: Spokane WA
Drifting
Joined: Jun 2009
Posts: 1,287
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From: Upstate NY
I installed a magnaflow high flow cat and what a difference -seemed better than ever.
Le Mans Master
Joined: May 2006
Posts: 9,226
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From: Irvine Ca
Plus there is no sense in having a cooler thermostat despite what you "think"....
And Engineers designed this really cool switch (pun intended) that controls the fan Automatically! The Driver merely need to control the direction and the speed of the car whilst this "cool" automated system handles the temps devoid of driver input......imagine.....
Last edited by jhammons01; Nov 11, 2012 at 12:01 PM.
Racer
Joined: Oct 2012
Posts: 294
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From: Spokane WA
Dude, don't give out bad info......if he has a 4x3 (which you haven't determined in this thread) and you put a 160° thermo in there, while he is going down the highway on cool days, and once his temps drop below 159°, his overdrive will kick out unexpectedly.....
Plus there is no since in having a cooler thermostat despite what you "think"....
And Engineers designed this really cool switch (pun intended) that controls the fan Automatically! The Driver merely need to control the direction and the speed of the car whilst this "cool" automated system handles the temps devoid of driver input......imagine.....
Plus there is no since in having a cooler thermostat despite what you "think"....
And Engineers designed this really cool switch (pun intended) that controls the fan Automatically! The Driver merely need to control the direction and the speed of the car whilst this "cool" automated system handles the temps devoid of driver input......imagine.....