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Username Post: 1990 GMC Sierra codes 33 and 34        (Topic#351976)
Posts: 35

Reg: 10-01-17
07-11-18 01:43 PM - Post#2739713    

I have a 1990 GMC Sierra 1500 5.7 L and at least once a week I get either a code 33 or code 34. I know 33 is low vacuum for the map sensor and 34 is high vacuum. I replaced the map sensor and the 2 codes keep coming up once and a while. And it is random with either code (it could be 33 one time and 34 another). It is not persistent however if I shut the truck off for a few minutes the code goes away and doesn’t come back for a week or so. I just want to figure out what could be causing it even though it doesn’t seem to effect how it runs or drives. it is an annoyance having the engine light come on because I have no choice but to check just in case if it is an “actual” problem. Thanks, Hawk

Senior Moderator
Posts: 28872
Age: 48
Loc: Texas
Reg: 08-01-03
07-11-18 02:53 PM - Post#2739724    
    In response to hawk1122

I haven't studied the conditions for those codes yet, but I'd start by ensuring the boot at the rear of the TBI isnt cracked or loose. Pretty common once they're this old. You can replace that boot and plastic hose assembly with regular vacuum line, just be sure to route it so it can't kink.

Also check your grounds at and next to the thermostat housing.


06 Silverado ISS / 06 Silverado SS / 06 300C SRT8

Posts: 35

Reg: 10-01-17
07-12-18 11:38 AM - Post#2739782    
    In response to someotherguy

I will check what you said but also just noticed that the gasket between the throttle body and air filter is pretty bad so this could be the problem.

"7th Year" Silver Supporting Member
Posts: 4156
Age: 55
Loc: Salt Lake City
Reg: 06-20-09
07-12-18 09:29 PM - Post#2739814    
    In response to hawk1122

It won’t be the cause. These aren’t like the newer trucks that have a closed intake past the MAF.

It could let dirt and moisture in, but it won’t cause the codes.


'95 K1500 Z71 EC Short Step 5.7L+0.040/NV3500
'00 Explorer XLT 4.0 V6 Auto
'94 K2500 5.7 NV4500 ECLB - SOLD
‘87 GMC S15 SCLB 4.3 Auto - SOLD

"The only thing necessary for evil to triumph is for good men to do nothing" Sir Edmund Burke

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