Engine Surges or "Pulses" with A/C AC on- another AC question

[1993GMCSierra]

Also, unless it's been converted (it will have a sticker either on the silver canister by the firewall, or on the compressor or radiator support saying "This vehicle has been converted to R134a" ), you're likely still driving with R-12 in the system. R12 was used up until the 1994 model year.

If it is, you cannot simply go to a store, buy a can of R-12 and "top it off". R-12 hasn't been sold since 1996, and these trucks do not convert to R-134a that well.

Your best bet is to get some R-12 manifold gauges off eBay, and a R-12 can adapter, and a can or two of R-12.

Check the pressures to see where you're at, the low side should stay around 28-32 with the A/C on MAX, and the idle around 1500. I would add freon until it's around there.


Is the air still cold? Do you have a thermometer to stick in the vents to see how cold?

 

[monstar_est87]

I have this same surge or pulse problem at highway speeds, but the A/Cs not running... Any thoughts..?

 

[great white]

I have this same surge or pulse problem at highway speeds, but the A/Cs not running... Any thoughts..?

Check for lean surge.

 

[monstar_est87]

Lean surge..? Can u elaborate please..? Never heard of that...

 

[great white]

Lean surge..? Can u elaborate please..? Never heard of that...

More commonly associated with carburetor fueled engines, it is still possible with fuel injection. Just the mechanism for the fault is changed, not the symptoms themselves.

Possible causes are fuel injector issues (weak, spray pattern, leaks, etc), fuel calibration issues (messed up "chip" tune), Mucked up BLM multipliers (messed up "Chip" tune), fuel pressure issues (pump, filters, regulators, etc), timing (missing counts, messed up "chip" tune, etc), air leaks in the intake tract (IE: vacuum leak, MAP systems are more affected by this than MAF systems), O2 sensor issues (old, contaminated, slow to respond, etc), running in closed loop, etc.

Basically, your engine is "leaning out" when in a cruise condition.

Sever cases causes "bucking", minor cases causes the engine to surge.

Out of adjustment TPS has also been know to cause similar conditions. I used to set them with a multimeter to be sure they were within OEM spec, even the supposedly "non adjustable" ones.

Best way to trouble shoot it is to hook up a datalogger and see what's going down while it's doing it with respect to long fuel trims and such and go from there...

 

[monstar_est87]

Wow thank u very much Great white ur the man... I'll get started on this and see wat I can come up with... Sorry to the OP for the thread jacking and slight of topic...