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Topic Review (Newest First)
02-05-2006 12:59 PM
re: Off idle driveability problems

Latest update:

Too cold and windy outside to do compression test.

I thoroughly cleaned distributor cap, rotor and cleaned all the distributor contacts.

I replaced the copper rotor pickup (from another distributor) as mine was worn very short arcing to the rotor.

I ran a new ground wire to the coil, back to the battery.

I had another old set of spark plug wires, from a V8, which I ?made? to fit my V6 plugs.

This made the increased the idle vacuum to steady 17 in hg in Park (1100 rpm), and steady 10 in hg in Drive/Reverse (600 rpm).

The engine easily goes to 4000 in park, with less hesitation than before.
Throttle response seems crisper with the timing at 19 BTDC.

The car is now at least drivable, drove it round the driveway. There is a definite misfire in gear, that?s not there in Park.

It still needs work, but my troubleshooting leads me to think the problem is ignition related. (Swapping carburettors has little effect). I now think I'm losing spark under load.
02-05-2006 04:04 AM
Originally Posted by 78transam
So just to clarify, is there a fault in the coil, (as I get no reading from BATT to Rotor Pickup, but get a reading from Ground to Rotor Pickup)?

Would this mean I need to replace it?

Thanks again.

Doc here,

Nope, the coil is fine, you just checked it differently..

02-05-2006 03:58 AM
re: Off idle driveability problems

So just to clarify, is there a fault in the coil, (as I get no reading from BATT to Rotor Pickup, but get a reading from Ground to Rotor Pickup)?

Would this mean I need to replace it?

Thanks again.
02-04-2006 06:15 PM
Originally Posted by 78transam
Doc, I just did another quick test on the coil.

Between the Black wire (Ground - I think) and the rotor pickup i measured 8000 ohms. Does this mean anything?
Doc here,

Yeah, 8000 is OK, your just bypassing the primary side to ground measuring it that way..

02-04-2006 05:55 PM
re: Off idle driveability problems

Doc, I just did another quick test on the coil.

Between the Black wire (Ground - I think) and the rotor pickup i measured 8000 ohms. Does this mean anything?
02-04-2006 04:02 PM
docvette Doc here,

If your getting no reading from the secondary side of the coil, you have found your problem! Although, I find it odd that it runs at all..but "Stuff Happens"

As far as the module goes, I think size wise they are different, but if It does fit, Electrically it should work..all it really cares about is pulses per second from the magnetic pickup..

Would there be other signs if the chain had jumped? The engine always hesitates on WOT, and some times pops through the carb.
Depends on how far it jumped..It could go from the a backfire on cranking, to no fire at all..That's why the need to verify everything is as it should be.

Go inside and stay warm..It's 75 and sunny here...

02-04-2006 03:30 PM
re: Off idle driveability problems

Thanks doc.

After getting no reading on the secondary side (with the DVOM on 10K) I tried all other ranges, even a different meter just to be sure.

I took the coil out of the cap to measure it. I have a spare ignition module from a V8 ?78 T/A, would that work in the V6? (I have no experience of ignition modules).

I don?t know how many miles the car has done, the odometer shows 80500, but the transmission needed to be replaced due to reverse gear failure. So I reckon its more like 180500. But I really don't know.

Would there be other signs if the chain had jumped? The engine always hesitates on WOT, and some times pops through the carb.

With my luck, and this car, I probably have ALL the problem you described, and will need a complete rebuild.

I will see how much more I can get done tomorrow, Its 34'F out, and raining. The firebird doesn't fit in a British garage. LOL.

Thanks again.
02-04-2006 02:52 PM
docvette Doc here,

Good Job on the troubleshoot!

First I'd be concerned about the Coil Secondary reading, It Should read between 6,000 and 30,000 Ohms, If your not getting a reading, you have a problem.

Are you sure you didn't forget to RESET your DVOM to R X 10K or auto range when you changed sides of the coil? If you forgot, It would read in error. If your Sure you had it right,

I Would bet the Carbon element has broken down inside the coil housing, and allowing just enough spark to fire at load loads (Idle)

Pull the Coil from the cap, and measure between the Red Wire and the spot where the Carbon element touches the coil..

If you don't get a reading, replace the coil...If you do get a reading, replace the element and the rubber gasket in the cap (you may need to buy a new cap)

As far as the module goes, no real practical way to test it yourself, If the Auto Stores in the UK don't offer this service, the only other recourse is to just replace the module..

If you do this, and no change is present, than put the old module in the NEW box and toss it in the glove box, never know when you might need it on the road..100 KM from nowhere.

If it solves the problem, Chuck it in the Round file..What ever you decide to do, If you remove and replace the module use heatsink compound! or it will soon die.

As Far as the compression goes, If you can do that , It can show the state of the timing chain..If the cylinders are low, like around 50 or 75 Lbs..your chain has jumped..This can happen anytime the car is over 100,000 miles..

The other way, is line up the Timing marks, pull the left valve cover, watch and VERIFY both intake and exhaust are Dead Closed, The Piston is at TDC, Compression..If you can't coordinate this, the chain has jumped..

If you have that, Then pull the cap, the rotor should be DEAD on #1 Plug tower..If not, or off a bit in either direction, Pull and inspect the gear and it's shear pin, be sure it's good, reset the dizzy, for dead #1 Plug tower, and try it again..

Seems to be narrowed Down to those three items..

02-04-2006 02:21 PM
re: Off idle driveability problems

Doc, I have been going through your list today, and thought I should post the results.

Firing order correct, Plugs are properly connected and well separated.

I can't get the ignition module tested as I live in the UK, I can't find any one that tests them. Is there a way I can test it myself?

Both advance units function properly. Can't see any wires shorting out. Distributor cap is clean, no cracks/carbon.

I carried out the Coil check, the Primary side (BATT & TACH) was 0.5 ohms. I couldn't get a reading on the Secondary side (BATT & Rotor Pickup). Meter indicated a broken circuit.

Could this cause my problem? Like the spark needing to jump the gap, causing week spark under load?

I rebuilt my old carb, and put it back on, just to see if it would make a difference. -Made no change.

I blocked off all vacuum ports, and have checked for intake leaks. Can't find any. The vacuum doesn't fluctuate, its 15in hg steady at idle.

I have a clear in line fuel filter, there is no problem with fuel volume, and the pressure is about 3.5 PSI, (cranking). Both filters are clean. Gas is new 95 octane.

There is an EFE system in the right exhaust manifold, it is stuck in the open position. All the emission equipment and control gear has been removed.

Have not done a compression test yet, I did check them last year and the results didn't indicate a problem.

There is no cat, and the exhaust seems to flow well at high RPM in park.

Thank you for your detailed responses.
02-04-2006 03:19 AM
re: Off idle driveability problems

Thanks for your input. I will do as you suggested and run through all the checks.
02-03-2006 09:05 PM
docvette Doc here,

Basics First..

Check AND Double check the plug wires, Vs Fire order.

1 6 5 4 3 2

Rotation is clockwise.
A reversed wire will give exactly the symptoms you describe..

Test the Module, Take it to the local Auto zombie and get it tested for free..several times , to heat it up. And if you need a new one, have that tested also before you leave..they have an abysmal failure rate out of the box..When reinstalling new OR old, do not forget the heatsink Compound, Or you will be doing it again a week or so later..

While your in there, make sure nothing is hitting as the advance plate moves and shorting out the module or magnetic pickup.

Check the star magnet for metal shavings, this will give false triggers to the coil..

Check both advance units, mechanical and vacuum, be sure both are free and working not rusted and shut or the diaphragm blown. you can move the mechanical by hand, and you can add 5 to 10 In Hg to the vacuum canister and observe the movement.

Check the cap for carbon spray from the rotor pickup inside the cap.

Measure the coil, With your DVOM, set for R X 1 , Calibrate the meter, (cross the probes, ( for a 000 reading, or make note of errors) Measure the primary, put one probe on the "Batt" terminal, and the other on the "Tach" terminal, It should read less than an ohm, but more than 000 ohms..much outside that range , replace the coil.

Next measure the Secondary side of the coil, set your DVOM for R X 10k ohms..put one probe on the rotor pickup, and one on the "Batt" terminal. It should read 6,000 to 30,000 ohms, outside of this range, replace the coil.

This will not detect internal arc~over on the inductor core windings, you need an oscilloscope for that, so if you suspect it, replace the coil with a known good one.

Inspect the coil well for water or condensation.

On a stock engine with no cam, 5 to 15 In Hg Vacuum is pretty low, It should be around 17 to 20 In Hg AND STEADY. Look for leaks , get some carb cleaner and spray it around the manifold With the air cleaner on..(you'll get false triggers) everywhere it speeds up you have a leak. Inspect all the hoses, and PVC for leaks/malfunctions. If the Vacuum is rapidly fluctuating, you have some valve/seal/seat issues..

Do a compression test, you should have 125 to 150 all the way around, +/- 10% ..If ALL are low, you may have timing chain Issues..

Do a Static timing setup..#1 Compression ,TDC, BOTH intake and exhaust Valves closed, timing marks lined up, (If you can't coordinate these, you have timing chain issues)and Dizzy DEAD on #1 plug tower. If you have the first 3 OK but the Dizzy is off, reset it to #1.

Do a Fuel pump Volume and pressure test, if outside spec, replace the pump. Replace the Fuel filters.

If it sat some time, drain and replenish the fuel, it could be full of water, or lost it's punch..

Look for bends, or restrictions in the Exhaust system, if found replace the pipe.

If you have an EFE valve in the right header, (closes on a cold engine until it heats up then opens) BE sure it is not stuck closed. 78 shouldn't have one though..

Do these basics and you'll find the problem!

02-03-2006 08:00 PM
Hope this helps....

Well everyone else has some pretty good ideas, however the problem you say only starts when you put it in drive?
Well if you have an auto, then check the detent cable, because if the engine drops and stumbles, then there is no way it will hold much vacuum. That cable is the only thing I can think of that would make the engine poop out in drive...
Also the only other thing I would do would be to check the fuel pump pressure. Besides that I have no idea.
Also you said there is gas in the egr valve block off, sometimes manual pump gas diaphrams break and leak gas into the engine, which thins the oil, check your oil also because if your oil thins it is pretty easy to get my your oil rings.
02-03-2006 07:28 PM
Jmark "If I SLOWLY increase the speed to 1000 RPM to keep it running vacuum goes to 0 in hg. The engine doesn?t want to go much faster than 1000rpm in Drive/Reverse. If I get on it too quickly it cuts out. It hasn?t got enough power to drive up the driveway. Put it back in park, vacuum returns to 15in hg."

Does the exhaust have a catylitic (sp?) converter? Sounds like its plugged from running so rich. Could have a bad exhaust pipe somewhere too. Seen a few crack inside and then fold over and block the exhaust. If you can, remove the exhaust from where the 2 come into one, ahead of the cat, and give it a try. Just might work.

02-03-2006 06:22 PM
matt167 leakey intake gasket I'd suspect, that will cause a vac leak, and you deffinetly have 1 of those.
02-03-2006 05:45 PM
Off idle driveability problems

Hello. I have been trying to correct the drive-ability issues with my 1978 231 v6 firebird, all stock.

Ever since I had the car, It has been undriveable, it couldn't idle for more a few minutes, backfiring through the carb on WOT and stalling coming to a quick stop. Vacuum has always been low and inconsistent, the plugs fouled very quickly, with black smoke under load.

I just replaced the carb with a re-manufactured Rochester 2bbl, and noticed immediate improvement in the idle, with 15 in hg vacuum @ 900rpm.

My problem is this:
When I put the transmission in Drive the RPMs fall to 550 and vacuum drops to 5in hg, the engine wants to stall. If I SLOWLY increase the speed to 1000 RPM to keep it running vacuum goes to 0 in hg.

The engine doesn?t want to go much faster than 1000rpm in Drive/Reverse. If I get on it too quickly it cuts out. It hasn?t got enough power to drive up the driveway. Put it back in park, vacuum returns to 15in hg.

I have checked for vacuum leaks, and can?t find any. I cleaned the plugs when I changed the carb. The plugs are still going black and are gas fouled, but not as bad as they were. Choke is working properly. The plug wires are separated, and I have checked (with a timing light) for a spark on all cylinders.

I tried swapping the HEI coil for another, no difference. Also cleaned and checked rotor and distributer cap.

I tried advancing the timing (the vacuum gauge pointed to late ignition timing). Its base timing is now at 19? at 900 RPM. Slight improvement in idle quality but had no effect on my problem. I am running new 95 octane gas, with two fuel filters, both clean.

However I noticed, under the EGR blank off plate, the intake port was full of gas. I don?t know why.

Sorry to be so long winded but I have spent a few days working on this and can?t fix it. I just want to give you as much info as possible, so as not to waste your time.

Any help with this would be very appreciated. Thank you.

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