Engine problems .... need help - The 1947 - Present Chevrolet & GMC Truck Message Board Network

71sub · 10-31-2002, 07:47 PM

I have a 72 Cheyenne LWB -- 350 / 700R4 w/ 3.08 gears.

I've changed the spark plugs, wires (loomed), cap & rotor, carburator, electronic control module ( HEI distributor ) and checked timing.

When driving and try to accelerate I loose power -- I get a pssssst noise comming from the right OR left side of the engine.

Also, I can't take any grades on the freeway above 40MPH w/o giving it too much gas -- otherwise it seems to sputter and pssst.

I'm running out of options --- it's less pronounced when I put the tranny in 3rd and NOT let it cruise in O.D.

Any help would be greatly appreciated.

Thanks

Ev

Michael A. K. G · 10-31-2002, 08:02 PM

That sounds a lot like my inline 6. I suspect a vacuum leak.

Might want to stick a gauge on there and see how it looks. Stockton is very nearly sea level, so you should be getting around 20 inches, steady (stock).

**COBALT** · 10-31-2002, 08:05 PM

Sounds like you have some vaccuum issues. Check that pesky PCV valve to make sure it's not leaking. Also, on your carb have someone rev the engine and spray some carb cleaner on the outside until suddenly your engine revs up. That's where your vaccuum leak is. Probably need a new gasket.

Um...are your sure your engine is getting enough fuel? Maybe your float is bottoming out too low...

jhow66 · 10-31-2002, 08:59 PM

Sounds like you have a restricted exhaust system i. e. loose stuff in mufflers. Try taking mamifold pipes loose and run it.

jhow66 · 10-31-2002, 09:02 PM

that is manifold

71GMC_3/4T · 10-31-2002, 10:05 PM

I can't exactly remember if your year of truck had it or not, but some of the V8s had a flapper thingy on the passenger side of the exhaust manifold exit. I think it had a thermal actuator that, when closed, forced the exhaust gas across through the intake manifold, warming the engine up quicker. I wonder if it's stuck in the closed position? What you're describing sounds like exhaust restriction, so if that isn't it, check your mufflers. If you had a catalytic converter, I'd suggest that, but alas, we ain't got no catalytic converters on THESE trucks!

71sub · 11-01-2002, 12:10 AM

THANK YOU for all your help !!!

I have an Edelbrock Performer RPM intake w/ Hooker headers and
newly installed Holley 750 w/ electric choke ....

I'm going to change the PCV valve and test for vacuum leaks.

I'll report back what I finally figure out.

Ev

cableguy0 · 11-01-2002, 12:12 AM

go get yourself a can of carb cleaner fire up the truck with it idling just spray aroung where ur gaskets are if it idles up theres ur vaccum leak. sometimes tightening up some bolts will fix it but if it turns out to be a gasket just get another. gaskets for sbc are very cheap no need to go through any hassle

*Piston71* · 11-01-2002, 12:28 AM

Take a reading from your spark plugs (pull a plug), if its black or sooted your running rich, if its white (which I suspect it will be) your running lean.

Based on what your describing, your dealing with lack of Air/fuel. Could be due to a failing fuel pump, internal problem in the carb (plugged booster venturi) or plugged exhaust.

If you have a muffler baffle thats broken loose, it will restict the exhuast bad enough to cause a decent idle but as soon as there is any flow involved, it simply wont have any power.

A good bit of info for us is going to be this: When this is happening (low power) is it smooth as if you werent even pushing on the gas? Or is it surging, power/no power/power? Or lastly, does it shake the engine as if there is a misfire?

Also let us know if you have a electric choke? Holley is VERY touchy on the choke's.

71sub · 11-01-2002, 12:53 AM

A lot of good information .... thanks again

The truck idles fine and well under acceleration but does that psssst noise when I try to get it too much OR more gas. Under load is when I get this problem -- when the hood is open and I'm pulling the throttle cable, I've never been able to re-create the problem.

Also, it will only happen in one spurt intervals -- the engine does not stop but you definately feel a loss of power. I lessen the gas pedal and just let the engine tell me how much gas to give it.

It's got an electric choke on a Holley750. I was getting the same problem on the old Holley600.

*Piston* · 11-01-2002, 01:06 AM

If I had this vehicle at the shop with the complaint you have listed... I'd be running a back pressure test on the exhaust system. Sounds like a text book case of restricted exhaust system problems.

Know anyone with a backpressure gauge? If not you can loosen the exhaust manifold to pipe bolts to allow a mid sized leak, OR remove the muffler. Test drive again to see if the problem has gone away. Make sure you dont have any crimped pipe in any of the system.

NOTE: Don't run it too long with the pipes loosened. I mean dont leave it like that.. You'll be fine while doing the tests.

71GMC_3/4T · 11-01-2002, 01:02 PM

Air filter? Fuel filter?
If you're running the stock breather, check to see if the snorkle is open when it's running.
My bet is still on exhaust, but I'm throwing other possibilities at you.

Michael A. K. G · 11-01-2002, 01:14 PM

You can check for restricted exhaust with a vacuum gauge, too.

Vacuum at 3000 RPM (after settle-down) should be steady and higher than vacuum at idle. Vacuum gauges are easier to find than backpressure gauges, and you don't have to tap the exhaust pipe.

Use manifold vacuum only (like the power brake booster).

I take back what I said earlier; your problem DOESN'T sound like my inline 6. The 250 doesn't idle very well.

10-31-2002, 07:47 PM	#1
71sub Registered User Join Date: May 2002 Location: Stockton, CA Posts: 169	Engine problems .... need help I have a 72 Cheyenne LWB -- 350 / 700R4 w/ 3.08 gears. I've changed the spark plugs, wires (loomed), cap & rotor, carburator, electronic control module ( HEI distributor ) and checked timing. When driving and try to accelerate I loose power -- I get a pssssst noise comming from the right OR left side of the engine. Also, I can't take any grades on the freeway above 40MPH w/o giving it too much gas -- otherwise it seems to sputter and pssst. I'm running out of options --- it's less pronounced when I put the tranny in 3rd and NOT let it cruise in O.D. Any help would be greatly appreciated. Thanks Ev __________________ 71 C/10 : 350 / 700r4 ; Ford 9" posi 3.50 with 31x10.5 rear tires, single drive shaft, C-notch, 2.5" drop spindles, keyless entry 71 Suburban 350/350 -- caddy tail lights, keyless entry, late model seats, billet Autometer instrument cluster 70 Ford Torino -- 351W (balanced & blueprinted ), C4, VDO gauges

10-31-2002, 08:05 PM	#3
COBALT Senior Member Join Date: Sep 2001 Location: Seattle, WA, USA Posts: 5,817	Sounds like you have some vaccuum issues. Check that pesky PCV valve to make sure it's not leaking. Also, on your carb have someone rev the engine and spray some carb cleaner on the outside until suddenly your engine revs up. That's where your vaccuum leak is. Probably need a new gasket. Um...are your sure your engine is getting enough fuel? Maybe your float is bottoming out too low... __________________ '69 3/4 ton C20 2wd-350ci/TH400 '69 3/4 ton Custom 20 2wd-350ci/4sp Manual '99 2wd 5.7 Chevy Tahoe Seattle, WA.

10-31-2002, 08:59 PM	#4
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Sounds like you have a restricted exhaust system i. e. loose stuff in mufflers. Try taking mamifold pipes loose and run it. __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

10-31-2002, 09:02 PM	#5
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	that is manifold __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

10-31-2002, 10:05 PM	#6
71GMC_3/4T Weapons Of Construction Join Date: Apr 2000 Location: Colorado, USA Posts: 1,095	I can't exactly remember if your year of truck had it or not, but some of the V8s had a flapper thingy on the passenger side of the exhaust manifold exit. I think it had a thermal actuator that, when closed, forced the exhaust gas across through the intake manifold, warming the engine up quicker. I wonder if it's stuck in the closed position? What you're describing sounds like exhaust restriction, so if that isn't it, check your mufflers. If you had a catalytic converter, I'd suggest that, but alas, we ain't got no catalytic converters on THESE trucks! __________________ 1971 GMC 2500, 402/TH400 4.10 Daily Driver Lafayette, CO

10-31-2002, 08:02 PM	#2
Michael A. K. G Registered User Join Date: Jul 2002 Location: California central coast Posts: 126	That sounds a lot like my inline 6. I suspect a vacuum leak. Might want to stick a gauge on there and see how it looks. Stockton is very nearly sea level, so you should be getting around 20 inches, steady (stock).

11-01-2002, 12:10 AM	#7
71sub Registered User Join Date: May 2002 Location: Stockton, CA Posts: 169	THANK YOU for all your help !!! I have an Edelbrock Performer RPM intake w/ Hooker headers and newly installed Holley 750 w/ electric choke .... I'm going to change the PCV valve and test for vacuum leaks. I'll report back what I finally figure out. Ev __________________ 71 C/10 : 350 / 700r4 ; Ford 9" posi 3.50 with 31x10.5 rear tires, single drive shaft, C-notch, 2.5" drop spindles, keyless entry 71 Suburban 350/350 -- caddy tail lights, keyless entry, late model seats, billet Autometer instrument cluster 70 Ford Torino -- 351W (balanced & blueprinted ), C4, VDO gauges

11-01-2002, 12:12 AM	#8
cableguy0 Senior Member Join Date: Apr 2000 Location: Delta,Pa Posts: 14,948	go get yourself a can of carb cleaner fire up the truck with it idling just spray aroung where ur gaskets are if it idles up theres ur vaccum leak. sometimes tightening up some bolts will fix it but if it turns out to be a gasket just get another. gaskets for sbc are very cheap no need to go through any hassle __________________ Owner of North Point Car Care in Dundalk Md. We specialize in custom exhaust on both modern and classic vehicles. We are a full service auto shop from classics to modern vehicles. Feel free to contact me with questions. I will give a 10% discount to any board member.

11-01-2002, 12:28 AM	#9
Piston71 Account Suspended Join Date: Oct 2002 Posts: 3	Take a reading from your spark plugs (pull a plug), if its black or sooted your running rich, if its white (which I suspect it will be) your running lean. Based on what your describing, your dealing with lack of Air/fuel. Could be due to a failing fuel pump, internal problem in the carb (plugged booster venturi) or plugged exhaust. If you have a muffler baffle thats broken loose, it will restict the exhuast bad enough to cause a decent idle but as soon as there is any flow involved, it simply wont have any power. A good bit of info for us is going to be this: When this is happening (low power) is it smooth as if you werent even pushing on the gas? Or is it surging, power/no power/power? Or lastly, does it shake the engine as if there is a misfire? Also let us know if you have a electric choke? Holley is VERY touchy on the choke's. Last edited by Piston71; 11-01-2002 at 12:33 AM.

11-01-2002, 12:53 AM	#10
71sub Registered User Join Date: May 2002 Location: Stockton, CA Posts: 169	A lot of good information .... thanks again The truck idles fine and well under acceleration but does that psssst noise when I try to get it too much OR more gas. Under load is when I get this problem -- when the hood is open and I'm pulling the throttle cable, I've never been able to re-create the problem. Also, it will only happen in one spurt intervals -- the engine does not stop but you definately feel a loss of power. I lessen the gas pedal and just let the engine tell me how much gas to give it. It's got an electric choke on a Holley750. I was getting the same problem on the old Holley600. __________________ 71 C/10 : 350 / 700r4 ; Ford 9" posi 3.50 with 31x10.5 rear tires, single drive shaft, C-notch, 2.5" drop spindles, keyless entry 71 Suburban 350/350 -- caddy tail lights, keyless entry, late model seats, billet Autometer instrument cluster 70 Ford Torino -- 351W (balanced & blueprinted ), C4, VDO gauges

11-01-2002, 01:06 AM	#11
Piston Account Suspended Join Date: Oct 2002 Location: Olathe, KS Posts: 2,300	If I had this vehicle at the shop with the complaint you have listed... I'd be running a back pressure test on the exhaust system. Sounds like a text book case of restricted exhaust system problems. Know anyone with a backpressure gauge? If not you can loosen the exhaust manifold to pipe bolts to allow a mid sized leak, OR remove the muffler. Test drive again to see if the problem has gone away. Make sure you dont have any crimped pipe in any of the system. NOTE: Don't run it too long with the pipes loosened. I mean dont leave it like that.. You'll be fine while doing the tests.

11-01-2002, 01:02 PM	#12
71GMC_3/4T Weapons Of Construction Join Date: Apr 2000 Location: Colorado, USA Posts: 1,095	Air filter? Fuel filter? If you're running the stock breather, check to see if the snorkle is open when it's running. My bet is still on exhaust, but I'm throwing other possibilities at you. __________________ 1971 GMC 2500, 402/TH400 4.10 Daily Driver Lafayette, CO

11-01-2002, 01:14 PM	#13
Michael A. K. G Registered User Join Date: Jul 2002 Location: California central coast Posts: 126	You can check for restricted exhaust with a vacuum gauge, too. Vacuum at 3000 RPM (after settle-down) should be steady and higher than vacuum at idle. Vacuum gauges are easier to find than backpressure gauges, and you don't have to tap the exhaust pipe. Use manifold vacuum only (like the power brake booster). I take back what I said earlier; your problem DOESN'T sound like my inline 6. The 250 doesn't idle very well.

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