86' - rough start, rough under load - any ideas?
Thread Starter
Melting Slicks
Joined: Jan 2001
Posts: 2,940
Likes: 5
From: Chesapeake VA
86' - rough start, rough under load - any ideas?
Hi All,
I have an 86 that has had issues, but they are getting worse. It has the following symptoms:
- long cranking time
- most of the time it fires and runs but I have to feather the gas until the engine coolant temp is around 110 degrees, then the idle speed varies but does not die
- yesterday it just sputtered and would not fire or start
- today it would crank up, but had the same feather the gas issue
- it seems to idle better when warm than cold
- Once it has warmed up, it drives ok if you use light throttle, more throttle = more shudder and hesitation
- going to WOT cause rapid small backfiring (like morse code, but backfiring)
- once warm it will have an occasional shudder and backfire at idle but it is much worse when cold
I have done:
- new injectors - one was ohm-ing at 11.2 when the rest were 16.1 so I just changed them all (not the cold start).
- new fuel filter - The filter was about 5 years old with +/- 20K miles on it, this removed some of the shudder and hesitation under load
- new ignition module - no change
- checked for codes - no codes
- checked plugs 1 & 3 but I need to check the rest
- checked timing
- Checked fuel pressure - 42PSI with the vacuum line disconnected. It will drop slightly when revving in park, but not substantially
Any ideas?
I have an 86 that has had issues, but they are getting worse. It has the following symptoms:
- long cranking time
- most of the time it fires and runs but I have to feather the gas until the engine coolant temp is around 110 degrees, then the idle speed varies but does not die
- yesterday it just sputtered and would not fire or start
- today it would crank up, but had the same feather the gas issue
- it seems to idle better when warm than cold
- Once it has warmed up, it drives ok if you use light throttle, more throttle = more shudder and hesitation
- going to WOT cause rapid small backfiring (like morse code, but backfiring)
- once warm it will have an occasional shudder and backfire at idle but it is much worse when cold
I have done:
- new injectors - one was ohm-ing at 11.2 when the rest were 16.1 so I just changed them all (not the cold start).
- new fuel filter - The filter was about 5 years old with +/- 20K miles on it, this removed some of the shudder and hesitation under load
- new ignition module - no change
- checked for codes - no codes
- checked plugs 1 & 3 but I need to check the rest
- checked timing
- Checked fuel pressure - 42PSI with the vacuum line disconnected. It will drop slightly when revving in park, but not substantially
Any ideas?
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
Connect your fuel pressure gauge to the fuel rail and check the reading when you turn on the key (don't start the car). Pressure should build to about 42 psi and hold. If it drops quickly, it is more than likely a fuel pump issue.
You can also tape the gauge to the windshield and drive the car. Watch the gauge (or have the passenger watch and record the readings). What happens as you start to accelerate or hold the speed at 50-60 MPH?
Have you checked to see if the injectors are firing? It may be issues with the electrical connectors to the injectors.
You can also tape the gauge to the windshield and drive the car. Watch the gauge (or have the passenger watch and record the readings). What happens as you start to accelerate or hold the speed at 50-60 MPH?
Have you checked to see if the injectors are firing? It may be issues with the electrical connectors to the injectors.
Racer
Joined: Sep 2010
Posts: 327
Likes: 2
From: se iowa
Check your tps? Mine went out and acted like it was flooding. Would only start when floored and then ran really rich-no idle and would die if I put it in gear. You can poke a multimeter in the top two wires with the engine warmed up but not running and the key in the on position. .54VDC is what you're aiming for i think.
Melting Slicks
Joined: Nov 2008
Posts: 2,765
Likes: 5
Everyone prior that has posted has great tips or areas to look into too.
While you are at it,
Check the Distributor Cap and Rotor.
Make sure the Dizzy Cap is not cracked on the outside.
On the inside of the cap check carefully around each brass or aluminum electrode (all 8).
Carefully check the center carbon brush contact to make sure it is intact and not worn or broken off.
Sometimes with High Output ignition coils and the stock center carbon
brush the surrounding plastic will burn up.
Low resistance aftermarket center brush electrode with new cap is the solution to that problem.
Sometimes the HEI Rotor center to outer electrode will break off or has burnt up entirely from wear and high mileage use.
The Ignition coil may be breaking down, shorting out internally also.
Hard to check at home.
Substituting a known good or new ignition coil is a good and acceptable troubleshooting measure.
Old Snap On or SUN Ignition oscilloscope is helpful for troubleshooting all ignition problems if you are familiar using them.
I use them still each at home.
But they are mostly lost troubleshooting tools from the past.
Brian
While you are at it,
Check the Distributor Cap and Rotor.
Make sure the Dizzy Cap is not cracked on the outside.
On the inside of the cap check carefully around each brass or aluminum electrode (all 8).
Carefully check the center carbon brush contact to make sure it is intact and not worn or broken off.
Sometimes with High Output ignition coils and the stock center carbon
brush the surrounding plastic will burn up.
Low resistance aftermarket center brush electrode with new cap is the solution to that problem.
Sometimes the HEI Rotor center to outer electrode will break off or has burnt up entirely from wear and high mileage use.
The Ignition coil may be breaking down, shorting out internally also.
Hard to check at home.
Substituting a known good or new ignition coil is a good and acceptable troubleshooting measure.
Old Snap On or SUN Ignition oscilloscope is helpful for troubleshooting all ignition problems if you are familiar using them.
I use them still each at home.
But they are mostly lost troubleshooting tools from the past.
Brian
Last edited by 87 vette 81 big girl; Mar 20, 2011 at 01:10 AM.
Instructor
Joined: Mar 2005
Posts: 216
Likes: 3
i have an 86 and had all those problems including leaky injectors, i spent a ton of money on it, and never did get it right till i put an edlebrock intake and a 670 holley on it and my troubles were over, plus picked up alot of hp with just as good of gas milage, the only reason i did that because 86,s are nortoriuos for intake problems
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