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My Grand Prix cuts off while in gear

  
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My Grand Prix cuts off while in gear

 
billonwheels billonwheels
New User | Posts: 11 | Joined: 04/12
Posted: 03/21/13
07:50 PM

I have a 1970 Pontiac Grand Prix with a 455 bored 30 over. 278 edelbrock cam with comp cam roller rockers. a 750 Edelbrock Carb and Edelbrock intake. I also have a 2500 rpm stall converter and the TH 400 has been rebulit with a shift kit. 2 questions....1. I've had my timing re-adjusted a few times and there is a hesitation when I give it gas. When I hit it it spits and pops a little and then picks up. Could this be a faulty Carb or fuel pump? It happened over night as the day before it ran awesome. 2. when I drive it for say 30 mins and I pull up to a stop sign it will shut off, but will crank right up in neutral or park. Any thoughts? Oil pressure gets down to 20psi when warm, never gets hotter than 180 degrees..  

70bird 70bird
Guru | Posts: 1064 | Joined: 02/13
Posted: 03/21/13
08:41 PM

Hello billionwheels

If it ran fine the day before and still runs fine once above 1/4 throttle as it sounds like you are saying my guess is it is carb failure. I doubt it is water in gas so maybe debris in idle circuit of carb from rust in tank etc.

I would check plugs, old wires arcing, points, condenser.

i also had a problem where the coil got weak when hot after about 10 minutes of engine running. Once it got hot it did this EXACT same thing. Try similar coil off another car.

Hope this info helps.  

idrivejunk idrivejunk
Addict | Posts: 4987 | Joined: 12/09
Posted: 03/22/13
05:09 AM

Sounds like the accelerator pump's diaphragm may have flipped over in the bore like Q-Jets do. That would mean you need a replacement one, its a cheap part. Just something you can check if you have the lid off the carb. Performer Series carb?  
idrivejunk

waynep71222 waynep71222
Enthusiast | Posts: 391 | Joined: 04/12
Posted: 03/22/13
07:11 AM

good advice above.. lets do some free and easy tests...

with the engine idling... WARM.. not hot   choke open...  use a mirror to look down the primary bores...  as you bump the throttle lever with your hand.  do you get an instant discharge from the nozzles that spray into the primary bores... this test can be done with the engine off also.. but you will have to hold the throttle part way open to clear the flood ...

this checks that the accelerator pump is working as designed if there is an instant spray from the nozzles.  if not.. the pump piston or one of the check valves could be held open by debris or debris could be clogging the discharge nozzle tips..


~~~~~~~~~~~

with a COLD engine...  disable your ignition system...  disconnect the fuel line at the carb..

extend the fuel line into a clean clear 2 or 3 liter soda bottle..   have a friend crank the engine for 15 seconds while you observe the pulses of fuel from the end of the hose..

you should get an almost instant start of the pulses.. each pulse should be full and equal to the others.. not diminishing..   in 15 seconds you should get about a pint of fuel.. (half a quart)

if you have a low pressure gauge.. 15 pounds..  hook that to the fuel line and crank the engine again..  then wait..  your fuel pump should hold pressure for 3 to 4 minutes.   if you have a 3 hose fuel pump.. it still should..

sediment in the fuel pump from various sources can hold the fuel pump check valves part way open reducing the ability of the fuel pump to build fuel pressure..  or to pull the fuel all the way up from the tank...

holes in the fuel lines or hoses can allow the fuel pump to pull air in.. instead of fuel directly from the tank..

any of these can cause issues at idle as the engine speed and pumping speed is reduced..

dirty check valves in the fuel pump can cause a vapor lock type of symptom.. as fuel when pressurized to 5 psi.. does not boil in the lines like fuel at 0 psi..    the only thing stopping it from boiling is the constant flow... when you slow to a stop.. the fuel does not get burned and it can heat up and expand in the fuel lines and pump.. causing the issues you describe..


a frozen or stuck diverter valve in the exhaust system can also cause issues if your car is equipped.. i don't recall if the 70 models had those ..  probably..  what happens is the exhaust is diverted too long across and under the intake.. boiling the fuel in the carb..  


lastly... and this is probably not your issue...

can you take a test light and hook it to the positive side of the coil..  ground the other end..

turn the key on..  as you slowly turn the key to the run position. the light should come on.. as you rotate and start the engine.. the light should stay on.. when you release the key to the run position.. the key should stay on..    GM and other ignition switches have burned away the contacts to the ignition system over the years.. allowing dead spots .. i worked on one car that when the owner stopped.. the big ring of keys she had swung and moved the ignition lock cylinder just enough to pull the contacts apart as they were burned.. that was a tough one to diagnose.. as it would stall at stop signs and instantly restart..


second lastly..

are you having any power brake booster issues.. that might be a blown power brake booster diaphragm or leaking internal valving thats causing a hiss down under your dash board..

power brake boosters can fail leaking huge amounts of vacuum when you depress your brake pedal.. enough to stall the engine.  

billonwheels billonwheels
New User | Posts: 11 | Joined: 04/12
Posted: 03/22/13
07:29 PM

yes Perfomer Series Carb.  

billonwheels billonwheels
New User | Posts: 11 | Joined: 04/12
Posted: 03/22/13
07:38 PM

I have notice a little leak from my brake booster...  

70bird 70bird
Guru | Posts: 1064 | Joined: 02/13
Posted: 03/22/13
11:17 PM

Hello

Did you have a chance to check the other items?

What type of leak at booster, fluid or air?

If its fluid it is the rear seal in your master, needs rebuild or new, new is best, suggest you monitor your fluid level. If it is air might be a dried rubber seal or dried cracked hose, just needs a new one, maybe silicone until you get a new one.

If your booster diaphram is leaking very much your brakes will feel harder than normal.

Either way I suggest removing hose and plugging end and see it it idles then.  

Bobs427 Bobs427
User | Posts: 66 | Joined: 02/13
Posted: 03/23/13
06:20 AM

Note this is for testing only: Leave the distributor curve the way it is for the time being. Don't touch it. It sounds like the timing is not set correctly. The timing has to be very late...With that said.

1.If you have a vacuum advance distributor, remove the vacuum source, set the base timing at 12* then set your total timing to 30* and have it all in at 2,000 RPM. Don't forget to reinstall your vacuum to the distributor.

2.If your engine responds to this you know it was a timing issue. Do that first before spending any money on parts.

3.If you think you have a vacuum leak. Use carb or brake clean. With the nozzle that comes with it spray around the intake manifold, base of the carb, vacuum booster, anywhere there is a vacuum source.

4.If you are using 10/30 engine oil your oil pressure could go to 20 psi at an idle. You can always try another gauge for piece of mind. 180* is perfect for your 455.

Bob  

ROWSLEY ROWSLEY
Guru | Posts: 981 | Joined: 07/11
Posted: 03/23/13
11:05 AM

i have had a bad hei module do the same thing! you can have them tested at the parts store.  sounds like dirty carb to me. how dose it run otherwise?  
76 455/4spd TRANS AM
69 GRAND PRIX 455/5SPD

Bobs427 Bobs427
User | Posts: 66 | Joined: 02/13
Posted: 03/23/13
09:08 PM

What you can do if your engine won't start. To check the Ignition Module try this.
Use good test equipment if you can afford it.

1.Hook the test light to the tach lead at the HEI Distributor Cap, and then to ground.

2.Now crank the engine. The test light should pulse, if the Ignition Module  is working properly.

3.If the Test Light stays on steady replace the Ignition Module.

4.Always run the test a few times.

5.Make sure you have good grounds and your battery has the proper threshold voltage. Because an ECM could also cause a no start problem...Thats why I said check and recheck... Grin  Cool

Bob aka-pepsi1  

Bobs427 Bobs427
User | Posts: 66 | Joined: 02/13
Posted: 03/23/13
09:11 PM

Sometime if your ECM is a problem, while the engine is running, Idle is fine. Just tap the ECM with your hand, if the engine quites, look for a bad connection and moister anywhere in the area of the ECM, if all that checks out okay suspect and replace the ECM.

Bob aka-pepsi1  

billonwheels billonwheels
New User | Posts: 11 | Joined: 04/12
Posted: 03/24/13
05:08 AM

I havent had a chance to check the bowls in the carb, the Brake booster is leaking a little fluid from master cylinder. I think I will try a Holley Carb.  

billonwheels billonwheels
New User | Posts: 11 | Joined: 04/12
Posted: 03/24/13
05:10 AM

The GP runs awesome otherwise. Oil pressure is at 20psi when warm and idleing. The engines tight.  

Bobs427 Bobs427
User | Posts: 66 | Joined: 02/13
Posted: 03/24/13
05:43 AM

heay billonwheels

If your using a 10/30 engine oil. (20) PSI at idle in gear is good oil pressure.

Bob aka-pepsi1  

Bobs427 Bobs427
User | Posts: 66 | Joined: 02/13
Posted: 03/24/13
05:56 AM

Icon Quoteidrivejunk:
Sounds like the accelerator pump's diaphragm may have flipped over in the bore like Q-Jets do. That would mean you need a replacement one, its a cheap part. Just something you can check if you have the lid off the carb. Performer Series carb?

"Happy Birthday MATT"

Thats a good point. You not only add the best for Paint and Body restoration. But your mechanical is great also. (Have you been hiding something)?...LOL...
I had forgotten about the Accelerator pumps turning or twisting or flipping over.
  I always use a little Vaselin on the bores. It will disapate as soon as gas gets to those parts.

Bob  

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