Low torque on a 5.7 94 GMC

[dreamingmuscle]

I have a 94 GMC 1500 with a 5.7. Heres what I done so far. Headers, tune up(plug wires etc), fuel filter. Punched out the Catalytic converter (no help there) Replaced it, and the O2 sensor. Cat back dual exhaust, BG 44K. Only code is 33 Map sensor. But the map sensor circuits check out fine. (So the shop says).

 

The truck has no pull It'll barely even break traction on a full throttle start It's not much better when you unload the tire on a hard corner. My sons wore out 93 4.3 S-10 pulls a trailer stronger then my truck will. It didn't used to be like this. Shoot it used though gravel just putting it in drive at a idle. Every shop I've taken it to says nothings wrong.

 

The transmission seems to be fine. Hits all the gears with a decent thump. RPM'S don't seem t be excesssive. At any speed

 

MPG is around 13 to 15 mixed driving. Crusing speeds are fine, but passing sucks.

 

I'm at my wits end on what to do. The only things that comes to mind is a new cam and lifters

 

Any ideas

 

Thanks Glen

[snoman]

I have a 94 GMC 1500 with a 5.7.  Heres what I done so far.  Headers, tune up(plug wires etc), fuel filter.    Punched out the Catalytic converter (no help there)  Replaced it, and the O2 sensor.  Cat back dual exhaust,  BG 44K.    Only code is 33 Map sensor. But the map sensor circuits check out fine.  (So the shop says).

 

The truck has no pull It'll barely even break traction on a full throttle start It's not much better when you unload the tire on a hard corner.  My sons wore out 93 4.3 S-10 pulls a trailer stronger then my truck will.    It didn't used to be like this. Shoot it used though gravel just putting it in drive at a idle.    Every shop I've taken it to says nothings wrong. 

 

The transmission seems to be fine.  Hits all the gears with a decent thump.  RPM'S don't seem t be excesssive.  At any speed

 

MPG is around 13 to 15 mixed driving. Crusing speeds are fine, but passing sucks.

 

I'm at my wits end on what to do.  The only things that comes to mind is a new cam and lifters

 

Any ideas

 

Thanks Glen

 

<{POST_SNAPBACK}>

 

 

 

 

Okay, first of all dual exhaust and a punching out converter will not help low RPM power, it will hurt it. Second, axle ratio and tire size will have a big effect on low end pull (if you have a lift with tires with stock gear no bolt on is going to fix that, you need gears) Third, I have a 89 4x4 burb that I bought new with same engine but it has a CAT still and a 3 inch single exhaust with a flow master and it has excellant low end responce and torque. I also use 93 octane and have for about 15 years now in it with base timing set at 8 BTDC and it really woke it up some. It was a slug on 87 when it was new, especailly on a hot day as the ECM will retard spark when it hear knock before you do and that will cut into power/responce a good bit. That engine has 9 to 1 CR and it will do a lot better on 89 or higher octane fuel. (I also gained 2 to 3 MPG on highway when I did this to my burb 15 years ago)

[dreamingmuscle]

Ok I understand the cat but i did replace it. The tires are a little shorter than stock (255/70/15.) There is no knock or ping but on my next tank I'll try 89 instead of 87. Timing is the factory setting I'm not sure off the top of my head what that is. I'll try adjusting the timing and see what happens.

 

Any more help would be great thanks Guys

 

Glen

[snoman]

Ok I understand the cat but i did replace it.  The tires are a little shorter than stock (255/70/15.)  There is no knock or ping but on my next tank I'll try 89 instead of 87.  Timing is the factory setting I'm not sure off the top of my head what that is.  I'll try adjusting the timing and see what happens.

 

Any more help would be great thanks Guys

 

Glen

 

<{POST_SNAPBACK}>

 

 

 

 

Factory timing is TDC. On tire size, while it is not too big, you may have a tall rear axle ratio which makes it tall. Look in glove box and see it there is a GT4 in there (3.73) which would be good for your useage and a GT5 (4.10) even better. If it is a GU6 it is only a 3.42 which will lack at times and then there is a GU5 (3.23) and a GU4 (3.08) which will limit it further. Dual exhaust though does reduce low RPM torque though contray to popular belief. Below is a link for a axle ratio/tire size calculator if you want to see how it all adds up.

 

http://www.snoman.com/HTML/axlecalc_5a.html

[dreamingmuscle]

I appreciate you help on this. Getting told nothing is wrong is getting real frustrating.

 

I don't think it's the tires. I've been running the same tire size since the truck was new. One thing I should mention though it was running rough and cutting out on me one summer so I took it to the dealer who said I needed a new Distributor. So I bought a rebuilt one installed it my self. And had a shop set the timing since I could not find the brown wire to disconnect that My Haynes manual said was on the fire wall. (turned out to be under the glove box.)

 

Any ways took the truck to the GMC dealer after I put in the new Distributor because it was stalling and running rough. Told them I put in new distributor and to check the timing since I could not find that stupid brown wire. A couple of days later I get a call Mr. Terrill we never seen a truck run so bad with out some codes coming up. We have no idea what to do. I asked did you check the timing. Oh no!! We can't do that its all done by the computer. Any ways I found a small shop with real and honest Mechanics that found my timing was 10 degrees retarded. There went all my faith in Dealer garages.

 

I didn’t notice how much power was gone until I had My fathers truck of the same year and engine size to compare it against. The results were night and day. Any way I can’t afford a new truck so I have to figure out this one.

 

How about the summit headers? What effect would that have on the low rpm torque?

 

Again many thanks

 

Glen

[snoman]

I appreciate you help on this. Getting told nothing is wrong is getting real frustrating. 

 

I don't think it's the tires.  I've been running the same tire size since the truck was new.  One thing I should mention though it was running rough and cutting out on me one summer so I took it to the dealer who said I needed a new Distributor.  So I bought a rebuilt one installed it my self.  And had a shop set the timing since I could not find the brown wire to disconnect that My Haynes manual said was on the fire wall.  (turned out to be under the glove box.)

 

Any ways took the truck to the GMC dealer after I put in the new Distributor because it was stalling and running rough.  Told them I put in new distributor and to check the timing since I could not find that stupid brown wire.    A couple of days later I get a call Mr. Terrill we never seen a truck run so bad with out some codes coming up.  We have no idea what to do.  I asked did you check the timing.  Oh no!! We can't do that its all done by the computer.    Any ways I found a small shop with real and honest Mechanics that found my timing was 10 degrees retarded.  There went all my faith in Dealer garages. 

 

I didn’t notice how much power was gone until I had My fathers truck of the same year and engine size to compare it against.  The results were night and day.  Any way I can’t afford a new truck so I have to figure out this one. 

 

How about the summit headers? What effect would that have on the low rpm torque?

 

Again many thanks

 

Glen

 

<{POST_SNAPBACK}>

 

 

 

 

While headers can enhance mid and upper RPM torque they can take away for low RPM power. It has to do with how fast the gasses are allowed to blow down/escape from cylinder as it is extracting the energy from it to turn crank. At low RPM's with open exhaust or headers it lets gas blow off too quick. Also it tends to lean mixture out as some excess hydrocarbon from unburbed gas get scavenged through and cuase O@ senso to lean mixture some more and reduce power. It is really a lot more complex at time on a closed loop engine than some think. It is not like the old carb days. Are you following me so far?.

[dreamingmuscle]

While headers can enhance mid and upper RPM torque they can take away for low RPM power. It has to do with how fast the gasses are allowed to blow down/escape from cylinder as it is extracting the energy from it to turn crank. At low RPM's with open exhaust or headers it lets gas blow off too quick. Are you following me so far?.

 

<{POST_SNAPBACK}>

 

 

 

 

Ok I can see the physic’s on how the back pressure of the exhaust would help turn the engine. I never thought of it that way because. I always thought that the compression and firing stroke is what turned the crank. I can also see where the o2 sensor would lean the mix in this situation.

 

Well there is a set of manifolds on eBay that I’m bidding on. They have a smog pump tube on them but I’m sure I can find a way to plug that.

 

I’m going back to stock forget this aftermarket crap its’ nothing but a headache.

 

BTW would running lean give the exhaust a smell? I thought mine smelled like unburned fuel or rich you might say. But never have been able to compare it with another vehicle or with another person who knows what their talking about.

 

Glen

[Mark_in_Tulsa]

Well I wouldn't ditch the headers. I'm guessing it's something in the Tranny, or maybe the TQ converter.

[snoman]

While headers can enhance mid and upper RPM torque they can take away for low RPM power. It has to do with how fast the gasses are allowed to blow down/escape from cylinder as it is extracting the energy from it to turn crank. At low RPM's with open exhaust or headers it lets gas blow off too quick. Are you following me so far?.

 

<{POST_SNAPBACK}>

 

 

 

 

Ok I can see the physic’s on how the back pressure of the exhaust would help turn the engine. I never thought of it that way because. I always thought that the compression and firing stroke is what turned the crank. I can also see where the o2 sensor would lean the mix in this situation.

 

Well there is a set of manifolds on eBay that I’m bidding on. They have a smog pump tube on them but I’m sure I can find a way to plug that.

 

I’m going back to stock forget this aftermarket crap its’ nothing but a headache.

 

BTW would running lean give the exhaust a smell? I thought mine smelled like unburned fuel or rich you might say. But never have been able to compare it with another vehicle or with another person who knows what their talking about.

 

Glen

 

<{POST_SNAPBACK}>

 

 

 

 

 

You are starting to get the big picture. The power stroke (after ignition) is driven by heat expanding the gas and forcing piston down. We like to think of it as a explosiion but it is a very quick burn when all goes right and the burn expands the gas to makes the motor turn, hence the higher tha CR, the more expansion that takes place before it is exhausted from cylinder. Modern engines are fairly well tuned from intake to exhaust but your TBI motor can respond to the right type of tuning. The things restricting it most in mid and upper RPM power is cam timing/lift and intake (manifold and TBI size) It lacks the tuned intake of the modular engines which can really boost power at certain RPM ranges that it is tuned for. With a warmer cam of proper grind and a different intake and TBI unt, it would be a completely different animal above 3000 RPM or so. On the smell, this is a tuff one because even lean can smell rich sometimes because a lean mixture may not burn all the way so it is hard to tell you exactally without sample. (it does have a different smell"flavor" than true rich though.) I am curiuos, what RPM are you showing at stall on these sluggish launches now?

[BigWheelinBubba]

My truck did the same thing, turned out to be a bad fuel pressure regulator.

[dreamingmuscle]

Don't know the RPM's (Don't have a tach) The launches are streight forward off the brake on the gas. No high RPM launches.

 

I've thought about fuel pressure problems but I don't know if the shop checked that or not. What's the best way to do that with out paying another $89 diagnostic fee.

[dreamingmuscle]

I also thought about going to get VA emmisions test done on the truck. I'm currently in the military and the truck is tagged in OK. So it doesn't require a emmissions test. How many questions would that answer??

[Wingnut]

That would depend on how extensive the VA emissions tests are.

 

I doubt it would help a whole heck of a lot though...And if you fail it, there's no telling what they might do.

 

Oh...And from personal experience, don't get caught with a radar detector in VA.

[snoman]

I also thought about going to get VA emmisions test done on the truck.  I'm currently in the military and the truck is tagged in OK.  So it doesn't require a emmissions test.  How many questions would that answer??

 

<{POST_SNAPBACK}>

 

 

 

 

You should have about 14 to 15 PSI on line from tank pump and 10 to 11 PSI after regulator. A plugged fuel fiter or bad pump would cause it to be weak at time but it would throught a code. (my 89 did many years ago when I had a filter problem)

[dreamingmuscle]

That would depend on how extensive the VA emissions tests are.

 

I doubt it would help a whole heck of a lot though...And if you fail it, there's no telling what they might do.

 

Oh...And from personal experience, don't get caught with a radar detector in VA.

 

<{POST_SNAPBACK}>

 

 

 

Nice to see some Okies chiming in. That is truck country out there.

 

Nothing they can do until I try to register it in VA. And the test is pretty extensive being so close to the Capitol.

 

I'm retiring from the army in July and if everything works out right (Read Job here) I'll probley stay here. If the job doesn't come through I'm headed back to OK where land is cheaper and the pace of life is slower. If I stay emmision is impossible to avoid so I might as well see what I'm up against.

 

Glen