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Username Post: TBI 350 backfires heavy throttle        (Topic#306209)
boogie 
Senior Member
Posts: 2126
boogie
Loc: Louisiana
Reg: 07-11-02
12-24-13 06:53 AM - Post#2410183    

94 GMC TBI 350. Same truck that is hard to start in previous thread.

This truck spits,pops,backfires through the throttle body under throttle pressure. I have replaced the fuel pump with an AC-Delco. New plug wires. I have tried a different TB and distributor. Runs great going down the road at steady throttle.

I noticed with a timing light that the timing retards when you apply heavy throttle which would/should cause a backfire. Computer bad? That is all I can think of.

Any ideas?
'85 GMC C1500 SWB 355 sbc


Edited by boogie on 12-24-13 06:55 AM. Reason for edit: No reason given.

 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 2922

Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
12-24-13 08:38 AM - Post#2410206    
    In response to boogie

Test the fuel pressure with a gauge, with the engine running.

It could have a broken fuel pressure regulator spring.
For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If the it has been altered from stock let us know about that too.


 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 2922

Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
12-24-13 09:05 AM - Post#2410211    
    In response to boogie

Another possibility is it has a flat cam lobe.

It would be a good idea to test the compression. The head gaskets tend to fail between the center cylinders as these trucks age, and if the gasket gets very burnt through it can cause pinging and backfiring.

Double and triple check the plug wires to make sure you did not cross any.
For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If the it has been altered from stock let us know about that too.


 
stumppuller 
"4th Year" Gold Supporting Member
Posts: 533

Loc: Canada
Reg: 11-01-04
12-24-13 12:52 PM - Post#2410276    
    In response to boogie

  • boogie Said:
I noticed with a timing light that the timing retards when you apply heavy throttle Any ideas?



That is normal at idle. The ECM runs a very high idle advance curve, and will retard the timing significantly during heavy throttle. It will then begin to advance the timing if the throttle is held open. If it didn't you would have severe detonation/stalling.

What is the history of this truck? Did it use to always run ok and then develop this problem, or did you purchase it this way?
-91 Sierra C2500, now K2500
-81 Chev K20


 
boogie 
Senior Member
Posts: 2126
boogie
Loc: Louisiana
Reg: 07-11-02
12-24-13 04:45 PM - Post#2410326    
    In response to stumppuller

No noise from valvetrain such as a loose rocker from a lobe going flat. Runs great on the road at steady throttle, give it some pedal and

it bangs pops thru the TB, back off and all is well.
It is worse in OD.

Just got it. Previous owner just installed a fuel pump. I figured he put an off brand in there so I put an AC-Delco in, same thing.

I put on a different TB, no help. I put a different(used) distributor in. I assume it is good, no change.

I have been working on these trucks a long time, never not been able to fix one until now.
'85 GMC C1500 SWB 355 sbc


Edited by boogie on 12-24-13 04:46 PM. Reason for edit: No reason given.

 
Allan In NE 
Frequent Contributor
Posts: 1296

Reg: 12-27-11
12-24-13 06:17 PM - Post#2410344    
    In response to boogie

Is this just one 'pop' or is it a quick series of backfires?

Allan
Lifelong GM automatic transmission specialist


 
stumppuller 
"4th Year" Gold Supporting Member
Posts: 533

Loc: Canada
Reg: 11-01-04
12-24-13 06:32 PM - Post#2410345    
    In response to boogie

Ok, so it's a "new to you truck".

I had a similar issue many years ago after I assembled my 350. Started and idled fine, very light throttle input no problem, but step on it and it would backfire badly through the Throttle body. I had adjusted the valve lash too tight (flat tappet hydraulic lifters, basically stock).
-91 Sierra C2500, now K2500
-81 Chev K20


 
bowtie44s 
Contributor
Posts: 853
bowtie44s
Age: 31
Loc: wv
Reg: 08-29-12
12-24-13 09:56 PM - Post#2410380    
    In response to boogie

Make sure your wires are 1-8-4-3-6-5-7-2 and make sure timing is set at 0┬║ or 2┬║ with wire unhooked. Do a compression check, if one cylinder is low readjust the valves, while you're doing that you can make sure no rockers or push rods are bent. If it still does it after all that you probably have a wiped cam. It happens often now especially with a bigger cam since they took the zinc out of oil.
Jeff

'88 chevy k3500 aluminum head 454
NV 4500

'84 chevy short bed dana 60 14bolt ff 383 stroker
44" boggers (rear) 44" tsl (front) hydraulic assist
welded front and rear 5.13 gears
th400 np205 twin stick -SOLD-


 
boogie 
Senior Member
Posts: 2126
boogie
Loc: Louisiana
Reg: 07-11-02
12-25-13 08:31 AM - Post#2410432    
    In response to Allan In NE

  • Allan In NE Said:
Is this just one 'pop' or is it a quick series of backfires?

Allan



Quick series of backfires
'85 GMC C1500 SWB 355 sbc


 
Chevytech 
Subject Matter Expert - Senior Member
Posts: 2922

Loc: Twin Cities, Minnesota, U...
Reg: 04-25-04
12-25-13 08:57 AM - Post#2410435    
    In response to boogie

A quick series of backfiring when cracking the throttle is an indication of a mechanical problem like a exhaust valve that will not open which could be a flat cam lobe, a burnt intake valve, or head gaskets blown between cylinders.

With the air cleaner off, earmuffs on, run the engine and momentarily crack the throttle wide open, letting go of the throttle before the engine builds much RPMs. If it has sharp popping through the intake, this is how a flat cam lobe for an exhaust valve shows up.

The engine does not have to have any valve train noise to have a flat cam lobe.

I pull the rocker covers and run the engine to look for a rocker that does not move much.

If the rockers all move well, then you should do a compression test.

I have seen spotless engines, with no sludge at all, with flat cam lobes. Read up on the changes made to engine oil, and ZDDP for more on the suspected cause of cam damage.

For those of you that are wondering why you are not getting replies to your thread:

Did you give the model, year, engine, fuel system type, and transmission information?

If the it has been altered from stock let us know about that too.


 
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