CTS trouble on 93 GMC Sierra 5.7

[Cotton90]

I have a 93 GMC K1500 5.7 that is running very rich to the point of black smoking when you rev it. It also has a random misfire at idle and the engine stutters as its shifting between 2nd and 3rd occasionally. I changed the plugs and made sure they were correctly gapped. my next thought on the running rich issue was to change my Coolant Temp Sensor but I noticed that mine is a single wire hookup and the replacement is a two wire. If were to change the sensor and pigtail, where does the second wire need to be ran?

I'm a diesel mechanic by trade so I'm not very savvy with these TBI engines.

Thanks in Advance!

 

[LTC8K6]

Are you sure there aren't 2 sensors? A single wire for the gauge and a 2 wire for the computer?

 

[Cotton90]

The one I located was between the first two cylinders on the driver side head. Is that not it? Like I said, I really don't know much about these rigs.

 

[LTC8K6]

I don't know either. I just know that sometimes there are two.

 

[Cotton90]

I just went out and checked and it does have another near the thermostat housing. I feel like a dumba$$! Anyone have a clue on the stutter between shifts??

 

[phucheneh]

There's another one near the thermostat housing. Screws into the intake manifold. I believe that is the actual ECT (or 'CTS,' as GM used to call it). The one in the head is for the temp gauge.

It's common that one is a single wire and one is a 2-wire, but I can't recall which is which. I want to say the one for the PCM in the intake is normally the 2-wire. The one in the head may have a two wire connector, but one of the pins is usually empty.

 

[LTC8K6]

Looking around the net, the single wire one on the head is for the gauge?

The 2 wire one for the computer is on the intake near the water outlet?

 

[Cotton90]

Could that sensor be causing it to run really rich?? It has a random miss at idle but its not terrible like a dead miss or anything. But when you "pat" the gas or rev it pretty quick then let off from an idle it starts missing pretty bad and sounds like it has a big cam in it for about 5 seconds then goes back to normal. It smells like raw gas coming out of the pipes and is getting about 9mpg on the interstate so I need to figure it out QUICKLY!! lol

 

[LTC8K6]

Could that sensor be causing it to run really rich?? It has a random miss at idle but its not terrible like a dead miss or anything. But when you "pat" the gas or rev it pretty quick then let off from an idle it starts missing pretty bad and sounds like it has a big cam in it for about 5 seconds then goes back to normal. It smells like raw gas coming out of the pipes and is getting about 9mpg on the interstate so I need to figure it out QUICKLY!! lol

Yes, it could be the culprit.

You can test it.

http://econtent.autozone.com:24999/znetcs/psb/en_US/2/0/113/test_sheet_su12.pdf

 

[phucheneh]

A more likely culprit is that the MAP sensor is hooked up to a broken vacuum hose. This will keep the reading for manifold pressure at or near atmospheric.

Being a very simple EFI system with no MAF sensor, all it has to go off of for airflow is how far open the throttle is (via TPS) and the MAP reading. And it's not exactly brilliant at correlating them. MAP at atmospheric means the helmet-wearing primitive PCM just goes 'WIDE OPEN THROTTLE WOOOOOOO' and dumps gas in like it's going out of style.

 

[LTC8K6]

MAP sensor should be at the rear of the passenger side valve cover.

 

[Cotton90]

I'll test the cts and see if is faulty. And is there a way to test the map sensor? The truck still has plenty of power. I use it to haul about 5,000 pounds of recreational vehicles and it takes it like a champ. But I'm still wanting to clear up the rich issue because you can't keep gas in the thing lol.

 

[master_shake_]

to test the coolant sensor you can use a multi meter.

insert the probes in the wires behind the plug and start the engine as temperature goes up voltage comes down. on a dc scale.

if it is stuck a 4.96 ish volts its stuck at -40C

map is the same way with amultimeter on the ground an signal return line the two that aren't gray.

snap the throttle and see if the voltage changes at idle should be about .96 volts. <-- might be higher.

if you have a multimeter go to your oxygen sensor on the left manifold and put the positive probe on the one wire and the black probe on ground.

on the dc scale make sure it is switch high to low i.e 900-100 mV.

if you find it sticks change it.

 

[Cotton90]

I'll try those things. Thank you!

 

[thedarkwolf]

I'd check the fuel pressure next. Could have a blown fuel pressure regulator.

 

[Cotton90]

Is there a port to check the pressure?

 

[master_shake_]

Is there a port to check the pressure?

on that truck no...

you can buy a fuel filter that has a test port should be about 11psi i think

you could very well be right about the regulator.

now that i think of it i had a truck in the shop that did the same thing.

 

[SyndromeOCZ]

You can rebuild the TBI in about an hour on those, the rebuild kit with all the gaskets, o-rings, ect are $25 at o-Reilly's, they were cheaper than auto zone and car quest.

Looks like everything else has been mentioned by other people.