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my 1984 crossfire c4 runs rough under load

 
blazefeva
I love my Chevy Chevette!
Posts: 1
Joined: 05/10
Posted: 05/25/10 09:33 PM

Hello All,

I just got a 1984 c4 which was not running at the time i got it.  fuel pump went out.  i have since replaced the fuel pump, fuel filter, all fluids and filters, all ignition components including distributor.  the car now runs and idles ok at all times but there seems to be a bug that i have not yet been able to pinpoint.  the car has 75k miles and is complete, nothing missing or modified.  the car is all stock.  what happens is under hard acceleration, at all speeds, the engine runs rough and studders.  it does accelerate and catches up speed but i know its not running 100%.  when cruising at freeway speeds, 55-80mph, with minimum accelerating the bug seems to clear out and vehicle runs smoothly.  its very noticeable with the sound of the exhaust note at all conditions mentioned.  does anyone have any thoughts or experience with this type of problem?  i appreciate any input and thank you in advance for all and any help.  

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waynep7122
I mow my lawn and find Chevys
Posts: 1114
Joined: 08/09
Posted: 05/26/10 09:57 AM

copy and paste this link

http://repairguide.autozone.com/znetrgs/repair_guide_content/en_us/images/0900c152/80/06/7e/6a/large/0900c15280067e6a.gif

print the chart... print several of them...

do you have access to a scan tool????? write the readings from the scan tool on the chart ... do it with the engine off.. cold.. key on...  
then again.. after you warm the engine up and have it running..

do you have a digital volt meter...  they are cheep... and needed..

set it to 20 volt DC scale...    start the motor .. turn on the headlights...

touch one lead to the negative battery post.. the other to the intake manifold..

touch one lead to the negative battery post.. the other to a chassis ground...

touch one lead to the intake manifold.. the other to a chassis ground..

the first reading should be around 0.04 volts.DC...

the second and third reading should be  0.02 volts DC...

if you get more than this...  you have a bad ground between those systems...

i would also like you to examine the ground wires on the top of the intake manifold..  you fill find the ones that ground the computer to the engine there.. they have to be clean and tight.. properly connected with a crimped on ring terminal..  remember the ECM is only grounded to the Intake/ engine ..



this i would like you to check with the scan tool....

the TPS voltage..   full sweep...   slowly.. look for steady increase in voltage.. then a steady decrease..  no drop outs... you can also do it with the digital volt meter. but its a bit harder as you will need the special gator clips that have piercing teeth...

how is the connector for the TPS... most of the 84s that i work on need to have them replaced.. the heat just cooks them..  till they crumble..  easy to replace..

have you updated the coolant temp sensor and the air temp sensor from the coaxial connector.. (round) to the oval connector with side by side pins.. used on the later models...  sure makes a difference..  


i would also like you to pull the coil cover off the top of the HEI...  examine the sides of the ignition coil...  is there any signs of white dust.. or burn marks on the sides of the black center near the external frame...

make sure that when the cover is off.. that you can see the ground strap that fits under the corner of the coil  and connects to the center terminal of the 3 wire connector up from the base...

i would also like while you are there... to find the 4 pin connector out of the distributer that connects it to the wiring harness.... make sure it is not close to the Spark plug wires..    the spark plug wires produce a high voltage corona..  this can induce voltage into this 1 and 5 volts circuits on this 4 pin connector....

you might also want to pull the cap.. check the rotor.. take a look at the bottom of the rotor..  they have burned through before..   it takes higher voltage to accelerate hard..  when the coil or rotor insulation breaks down.. it can make misfiring problems like you describe..

i have also found bad pick up coils.. loose or worn capacitor harnesses from the module to the cap ..  check the hold down screw on the 3 wire harness.. that is the spark return for the high voltage.. if its loose.. you may have a mis fire..   most parts stores carry replacement capacitor harnesses.. under 20 bucks..    the 4 wire harness is harder to find..

i surf through this site daily.. post what you find...

if you have an OLD SCHOOL type of repair shop... they might still have a big scope that has an adaptor for the coil in cap HEI .. have them scope the ignition ..

and how long has it been since you replaced everything...

did all the  spark plug wires pop down hard????  double check them.. i have found many loose at the cap...

if the contacts are burned at the cap...  you will have to replace the cap and wires again..    spark plug wire separator kits are available for that model from various corvette suppliers..  its important...  i think they were like 20 bucks..   exact duplicates..  not aftermarket designs...  

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sniff
I love my Chevy Chevette!
Posts: 3
Joined: 08/10
Posted: 08/10/10 07:12 AM

i have the same mileage and same problem, replaced valve covers and gaskets, air filter, pcv hose, took passenger side thermac hose and added vacuum gauge which draws 17 inches, idles ok at 600 rpm, real rough under load, will check distributor and maybe change fuel filter next  

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yazzoos
I love my Chevy Chevette!
Posts: 3
Joined: 08/10
Posted: 08/28/10 04:46 PM

84 corvette crossfire starts but cannot set timing. unplugged timing advance for computer, have to rotate distributor 1 1/2" to make engine even run decent. but about 3 minutes later, the exhaust manifolds are cherry red.

replaced fuel filter, fuel pump, checked timing chain, new coil, new magnetic pickup, new module,
2 new idle air controllers, timing cover has been replaced and has a bolt on pointer. distributor is set at top dead center pointing to #1 plug wire.

any ideas

could waynep7122 read this over and have any suggestions  

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waynep7122
I mow my lawn and find Chevys
Posts: 1114
Joined: 08/09
Posted: 08/28/10 06:11 PM

ok... lets start with some basics..

there are 2 timing areas for chevy... 12 o clock and 2 o clock...  they use different dampers...  they use different timing pointers...

the keyway in the center of the damper is always aligned with one of the 3 crank pulley bolts...     if the timing mark on the damper is at TDC... with the pointer... if the correct damper and pointer are installed... one of the 3 pulley bolts will be 45 degrees to the drivers side of center...


notice the crank woodruff key in this picture... 45 degrees to the right...

Sbccamtiming


notice that in this image of the harmonic damper.. that one of the 3 pulley bolts lines up with the key way...   i am not even talking about the outside of the inertia ring mark...

Chevydamper

so... it the timing mark on the damper is at close to 12 o clock .. or close to 2 o clock.. see where the pulley bolt is...



why am i telling you this...  if you get the timing pointer or the harmonic balancer mixed up.. you will have to turn the distributer that equates to about an inch and a half on the harmonic balancer...

normally people pull a number one spark plug.. work the crank backward... till it hits a stop threaded into the spark plug hole...  the damper marked..    then the crank rotated forward until it comes to a gentle stop again..   marked again...   the gap divided... thats TDC...

my way..  only really kinda needs a mirror to view the pulley bolts..


when i drop in a distributer... i set the timing marks to what ever i want base timing.. then i line up the reluctor tips and the pick up coil tips.. and lock the housing.. after i make sure its down on the oil pump shaft..

this gives me almost exact base timing without a light..   good enough that i can start the car by just reaching though the open window after sticking the distributer in..  makes me look like a really great mechanic...


i do try to read as many posts as i can.. but sometimes i am over on several others forums.. or watching tv.. or out fixing cars... junkyarding... shopping...  i actually don't work for the mag.. i just surf here ... and try to keep as many corvettes running as i can..



oh... as for some other things...  check the Hose to the MAP sensor.. little black box  with a green 3 wire connector... and a vacuum hose to it..   it gets manifold vacuum.. the hoses are usually rigid plastic.. and a few have cracked... and the rubber hoses split...

do you have access to a scan tool....  so you can look at the live data stream.. make sure the sensors are feeding the proper data to the ECM...

one thing...  you replaced the ignition coil....   you replaced the pick up coil....

any chance the coil in the cap wires got reversed...  or the pick up coil might have been the wrong one...

it is possible that the module is firing the coil between the cap terminals..   this requires some examination of the spark plug wire terminals inside the cap...   the aluminum squares that stick down from the top terminals..

see if the arc trail is even across the whole blade...  or all the way to one side....  

you have to understand. the high voltage jumps 2 gaps... inside the cap... and at the spark plug....  

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yazzoos
I love my Chevy Chevette!
Posts: 3
Joined: 08/10
Posted: 08/28/10 06:22 PM

appreciate all the info. going out to garage to give it a try. thanks so much.  

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yazzoos
I love my Chevy Chevette!
Posts: 3
Joined: 08/10
Posted: 08/31/10 08:50 AM

hay wayne thanks so much, she's running great! went over all you told us and it was all good.set timing and found that the new air intake controllers have to reset in computer at 35mph. he jacked up wheels to rev up and the controllers reset themselves and engine regulated. went back and set timing again. took it for a run. super thanks again  

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magic2756
I love my Chevy Chevette!
Posts: 2
Joined: 08/11
Posted: 08/28/11 07:55 PM

Hello I`m trying to ask a Question about a 84 Vett also can`t find the place to post on this site .
you can email me at bestfriend2765@yahoo.com
Thanks for any help : I`m have timing issues thanks again  

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