93 C1500 Stumble Off Idle Condition

[silverado1993]

OK guys, I've sat here for 3 consecutive nights at work searching old threads trying to figure this one out, but I'm gonna have to ask for some help if possible.

I have a 93 C1500 with a 305 engine in it. Just got it back out of the shop last Friday for a ton of stuff that I'd been putting off. Here is what all I had done...

1. Intake manifold leak repaired
2. New distributor
3. Full tune up (AC plugs, best wires, rotor, button, etc.)
4. New radiator
5. New TBI gasket (precautionary)
6. New TPS sensor

That pretty much takes care of the engine side of things, the rest of the work was brakes, front end work, etc.

I'm still getting a slight off idle stumble when gently accelerating, and once I get on the throttle a bit more it pulls out of it. Otherwise the truck runs really good now. I do notice when I hold steady throttle at around 40-45, there appears to be either a rough run or a miss, and it feel it transmitting back thru the driveline in the form of a "clunky" sound.

He put it on the rack and checked for any driveline lash while I watched, and says that it's within normal specs so that's not the issue. I asked about the EGR valve/solenoid, he says he pulled vacuum on the valve and it held, and that he's never had to replace a solenoid.

I'm pretty much stumped at this point. Prior to taking it to him, I had already replaced my coolant temperature sensor. Seems like something really minor to me, but it's just avoiding me at this point. I keep wanting to come back to the EGR though.

What do you guys think?

[Zredneck94]

Did your full tune up include a new o2 sensor?

[silverado1993]

No, looking at the ticket I don't see that. Think that could be the issue?

[someotherguy]

Don't assume that EGR valve test proves a good valve. If it's opening too easily, or partially blocked open because of a chunk of carbon crud on the seat, it can absolutely cause the problem you're describing.

You could try unplugging the vacuum line from the solenoid to the EGR and plugging the line with a golf tee, that may at least rule out the EGR opening too easily. Or, fashion a quick block-off plate and try running that way for a test drive, if the stumble goes away, replace the EGR - and only replace it with one from the dealer, chosen using your VIN, or at the absolute minimum a Delphi brand. Avoid the one-size-fits-many parts house brands. TBI engines are very picky about the EGR.

Richard

[silverado1993]

On the EGR, when is vacuum present on that line and when is it not? The reason I ask is that at idle, I can pull the hose off and I feel no vacuum on the hose. Should there be vacuum there at idle speed or does the EGR circuit only work at other RPM's?

I appreciate all the input and other than this one what I'm calling "minor" glitch after all the work I had done to it, the thing runs like a champ. Just gotta get this hiccup out of it.

[someotherguy]

Testing past the solenoid you should not have vacuum to the EGR at idle. That's normal. EGR comes into play once you start getting into the throttle a little bit, to cool down the combustion chamber with inert exhaust gas, which allows for a more aggressive timing curve, without detonation.

Richard

[silverado1993]

OK I'll plug that line off and do a test run tomorrow and see what I come up with. Thanks for your help!

[oldred95]

If the fuel pump is original I would really check the fuel pressure and even if its not original I would still check it. The thing is on TBI there is no test port. The GM tool goes in place of the fuel filter and has a test port on it. The reason I say check the pressure is because that truck is 16 years old and I've seen multiple trucks have weird stumble rough running issues and all have ended up being low fuel pressure because the hose between the pump and sender is rotted and leaking. On TBI fuel pressure is everything. They only run around 10 psi so if you are at 7 psi its not going to be right. When the hose is rotten and leaking they tend to all run right about 5-6 psi.

A stuck EGR valve will cause roughness at idle and once you come off idle you won't notice it unless its stuck wide open.

If it does it on cold startup as well as a hot engine you can kind of rule out a 02 sensor causing it since they don't work until they warm up to a certain amount to go into closed loop.

[someotherguy]

Agreed; it's never a bad idea to check fuel pressure when troubleshooting an EFI engine.

Richard

[02Z]

Originally Posted by oldred95

If the fuel pump is original I would really check the fuel pressure and even if its not original I would still check it. The thing is on TBI there is no test port. The GM tool goes in place of the fuel filter and has a test port on it. The reason I say check the pressure is because that truck is 16 years old and I've seen multiple trucks have weird stumble rough running issues and all have ended up being low fuel pressure because the hose between the pump and sender is rotted and leaking. On TBI fuel pressure is everything. They only run around 10 psi so if you are at 7 psi its not going to be right. When the hose is rotten and leaking they tend to all run right about 5-6 psi.

A stuck EGR valve will cause roughness at idle and once you come off idle you won't notice it unless its stuck wide open.

If it does it on cold startup as well as a hot engine you can kind of rule out a 02 sensor causing it since they don't work until they warm up to a certain amount to go into closed loop.

I don't think he mentioned changing the fuel filter either...that would probably be a good idea.

[oldred95]

Originally Posted by 02Z

I don't think he mentioned changing the fuel filter either...that would probably be a good idea.

[silverado1993]

Yep, I did the fuel filter myself prior to it going into the shop.

[silverado1993]

Originally Posted by oldred95

If the fuel pump is original I would really check the fuel pressure and even if its not original I would still check it. The thing is on TBI there is no test port. The GM tool goes in place of the fuel filter and has a test port on it. The reason I say check the pressure is because that truck is 16 years old and I've seen multiple trucks have weird stumble rough running issues and all have ended up being low fuel pressure because the hose between the pump and sender is rotted and leaking. On TBI fuel pressure is everything. They only run around 10 psi so if you are at 7 psi its not going to be right. When the hose is rotten and leaking they tend to all run right about 5-6 psi.

A stuck EGR valve will cause roughness at idle and once you come off idle you won't notice it unless its stuck wide open.

If it does it on cold startup as well as a hot engine you can kind of rule out a 02 sensor causing it since they don't work until they warm up to a certain amount to go into closed loop.

I know he didn't do a fuel pressure check, so that needs to be on my list to go back over. As far as cold vs hot occurrences, it runs great cold, but when it's up to operating temp is when I notice the just off idle stumble and the bucking in the driveline at cruise speed with just a maintained foot on the pedal. That is mostly in a small incline when I may back out of the pedal just a bit then I notice the bucking along with the rough running at that RPM range.

[silverado1993]

Appreciate all the ideas, I'm getting my list together of things I need to double back and check on that got overlooked when it was in the shop.

[silverado1993]

I'm embarrassed! Got home from work this morning and tore into it and it was all my fault. A couple of months ago I pullled the EGR off to see if it was carboned up. When it wasn't, I reinstalled with a new gasket but I guess I failed to get the inner nut tight and it was practically only finger tight this morning when I went to pull it off. Sooooo.... I tighten it up really good, also found a very large, yet very loose fitting vacuum line at the rear base of the throttle body. Replaced that and went out for a test for grins. Low end stumble is gone! Like I said, I'm embarassed to even admit it but it can be the simplest of things sometimes..

I do still have the "bucking" when just maintaining throttle and it occurred to me to ask since I'm running a Flowmaster with no cat, could a lack of backpressure be causing that maybe?

[02Z]

Originally Posted by silverado1993

I'm embarrassed! Got home from work this morning and tore into it and it was all my fault. A couple of months ago I pullled the EGR off to see if it was carboned up. When it wasn't, I reinstalled with a new gasket but I guess I failed to get the inner nut tight and it was practically only finger tight this morning when I went to pull it off. Sooooo.... I tighten it up really good, also found a very large, yet very loose fitting vacuum line at the rear base of the throttle body. Replaced that and went out for a test for grins. Low end stumble is gone! Like I said, I'm embarassed to even admit it but it can be the simplest of things sometimes..

I do still have the "bucking" when just maintaining throttle and it occurred to me to ask since I'm running a Flowmaster with no cat, could a lack of backpressure be causing that maybe?

"Bucking" sounds like a skip, but you say you've changed all ignition parts...soooooo, I'm stumped too. I don't think it's got anything to do with backpressure.

[silverado1993]

My mechanic isolated the bucking at certain speeds to the torque converter getting weak. Gonna run some Dr. Tranny Shudder Fix stuff in it and it's a bit better, so it's time to save up the funds and just do the tranny when we go to replace the converter.

[silverado1993]

Thing still is just doggy on throttle just off idle. Got good spray patterns in the TBI, been all over checking for vacuum leaks, can't find any others. Don't quite know what to make of it. I keep suggesting fuel pump to my mechanic but he's not sold on that yet.

[para driver]

just for giggles, I tuned my distributor at idle to max vacuum, and it really smoothed out my 'flat spot'.. I think the engine still wants more fuel, but the timing helped.

[someotherguy]

Dialing in timing based on vacuum might work OK but I'd prefer to do it by numbers. Factory spec on a TBI small block is zero, with the tan/black wire disconnected. Up to 2-4 degrees of advance is pretty safe for a small power boost. If it doesn't run right at that setting, you have other issues that need to be addressed.

Richard