Slight engine surging.

[muncher]

Hi All. I will try to describe as best I can my issue. I have a V8 305 GMC Sierra. It has fuel injection even though it looks like a carb. When engine is cold it drives smooth. However when it is warmed up I notice that when I am maintaining a constant speed. The engine feels like it surges slightly. Almost like I am accelerating on and off lightly. (Hope that makes sense). If I accel or back off it is fine. Only happens when warm and only when I maintain a constant speed. To me it feels like a vacuum leak? All the US smog equip is connected. We don't need that here so if that's faulty I can disconnect it.
Hope you can help............Cheers.

[geezer#99]

Take a pic of your motor.
We might see something amiss.
Does it have the stock dizzy?

[craig113]

? ? Do you have the defrost in the on position? It could be the compressor kicking in to defrost the windows and take the moisture out.

[geezer#99]

Originally Posted by craig113

? ? Do you have the defrost in the on position? It could be the compressor kicking in to defrost the windows and take the moisture out.

Do you think he really needs to defrost his window when it's in the 70's??

[muncher]

Craig no, defrost is off. I will get a pic of engine up. As far as I can see everything that is supposed to be connected is. Cheers

[Cabnchassis]

If it's throttle body you probably don't want to go through the hassle of a smog delete unless you're pretty serious about that truck. I bought mine with the egr plugged, no oxy sensor and no cat. Threw a lean condition code constantly along with running like crap. The ECU in TBI trucks makes all the rules and you have to play by them or have it re programmed.

The ECU in these trucks is pretty bossy and if any of it's diagnostic tools don't work just right you'll have fuel/air mixture issues because it will be making it's adjustments based on faulty diagnostics and therefore making *** backwards adjustments. . .

If the CTS sensors are bad they can cause vacuum leak like issues. There are two the one for the ECU to go by and one for the gauge. THey are pretty cheap and if failed can cause big problems, I consider them a wear item on these trucks. I had the gauge sender fail on a 454 tbi work truck and ended up ruining the heads and later blowing the motor (the boss said push it till drops...)

The reason they are important though is because if the ECU believes the engine is running cold it will over fuel the engine to save possible wear and tear.

The defroster matters because these trucks are "climate controlled" and the a/c will cycle whenever the defroster is on in order to draw moisture from the air faster. When the ac cycles these trucks get kicked in the pants and not in a good way, so it was worth mentioning, even if it's warm where you are since a lot of people don't realize the ac kicks for heat too.

TBI=throttle body injection and TB is the throttle body itself so I don't confuse you.

The throttle body system loves to leak vacuum, but is very simple as far mechanical principles go. With a can of starting fluid (ether)Spray the base of the TB (there is a gasket under it and it's held in by 3 oddly spaced bolts and likes to cause trouble) and around the intake manifold gasket and any other vacuum point or line you see. Keep in mind that if it is a vacuum leak at the intake it can be located in the intake passages internally. The gaskets can cross over and allow the engine to pull air from the cooling passages and generally makes the truck run like junk. Usually it'll cut out or stumble at low to mid rpms for major vacuum leaks though

Make sure the gaskets are installed below the throttle body riser and below the air cleaner. Basically it amounts to one on top and bottom of the riser. If you pull them the truck will sound fine, but when you put them back in you'll notice the ECU cares that they are installed and working. I rarely find the one under the air cleaner on old trucks.

Located on the passenger side of the TB is a "sensor" looking thing that can be removed with an 1 1/8" wrench or a large cresent. It's actually the Idle Air Control valve or the "IAC" as you'll hear it called. you can remove it and clean it with carb cleaner (spray the **** out of it around the pintle), just don't push on the pintle because you can damage it. Diagnosing this one is a little tricky though. When you re install the IAC you need to either turn the truck on and off 3 times in a row or let it idle for about 10 minutes. This resets the connection between the ECU and IAC. The truck will probably run perfect all of the first day or until it's parked for a few hours. When you reset the IAC connection the ECU will usually make adjustments that solve all of the fuel mixture problems. When you park it for the night and start it the following day it will run like trash again if the IAC is failed or is not part of the problem. Basically to know for sure it was the IAC causing the problem you'd need to either replace it outright or clean it and see if the truck runs better for a few days in a row. Usually it will idle poorly or stumble at low rpms/idle if its failed or dirty but I honestly don't know for sure if it operates through the entire rpm range or not. When one of these TBIs acts up I try to check diagnostic equipment failure first, since the engine lives off it and will make adjustments mechanically that can make it hard to diagnose anything on the truck.

Oxygen and MAP sensor can also cause weird issues with idle/throttle/mixture since the ECU relies on all of that info coming back clean so that it can make good adjustment. Plugged EGR valves can do strange things to a motor too.

Low fuel pressure or vacuum leaking at the fuel pump also cause TBI spaz out fun.

THe question about your distributor is pretty relevant too, is it stock?

Also what elevation are you living and driving at and what elevation did you buy the truck at? There is a different timing setting for high altitudes.

[muncher]

cabnchassis. wow thanks for the detailed reply. Because I also have a low speed issue I wonder if the IAC is faulty. When I slowly drive into a parking space ie just touching accel, revs just above idle. The engine splutters and almost wants turn turn off. When I feel it's going to do that I give it a little more power and all is good. What do you think?
I am going to check everything you highlighted ...........cheers

[Cabnchassis]

Happy to help. Someone literally walked me all the way through the system one parts run at a time, or I would have spent a ton fixing it or broke it and spent even more swapping it. After doing all that though its impressive how well it runs for 210k and 20+ years. I tried really hard to justify swapping it to diesel but I just couldn't find anything at all to complain about with this motor. Except that this kind of irritating crap can't even happen to a detroit :