350 rough idle when cold, PING

amd103 · April 29, 2002

Hi folks,

Recently bought a new to me Suburban. 1990, FI, 5.7L, 85k on the rebuilt 350. Runs pretty well with decent gas mileage. I've recently replaced the EGR valve, plugs, wires, and thermostat. I've also run some injector cleaner through the tank a few times. The EGR valve took care of a "check engine" light problem, but nothing seems to clear up a rough idle when the engine is cold or heavy ping when the engine is hot and pulling hard on a grade.

Although it runs when cold started, the engine "hunts" in RPM and engaging the tranny (700r4) will occaisionally cause it to die. No problem when it's up to temperature, although it does 'hiccup' every once in awhile at a stop light or something. I've also noticed some fairly heavy ping when pulling a steep grade at speed (i.e. the throttle is probably 50% plus).

What are some possible "usual suspects" for this sort of thing? I like to tie things up in a neat package...is it possible the problems are related? Could this be a timing problem or are we looking at a sensor malfunciton? (nothing showing on error code checks).

Thanks a lot!

95Yukon · April 30, 2002

check and/or replace the cap and rotor if you haven't already, then check the spark timing with a timing light. Check the sticker under the hood for procedure on this, can't remember if there's a wire to be disconnected before timing it or not. Other than that, it sounds to me like it could be the throttle position sensor (which if I remember correctly are "adjustable" on those..you can loosen it and rotate the mounting point to compensate a little) also, check for any vacuum leaks. Sometimes the pcv valve will get stuck and cause a straight vacuum leak you can't hear because it sucks air in from inside the valve cover. Double check all the rubber boots going to the TBI, they tend to crack with age. Other than that, it may be just worn injectors or a fuel pressure problem, but check the cheaper stuff first.

Is it running within the normal temperature when it is pinging? Have you tried some 93 octane just to see if it reduces it some? High engine temperatures can cause pinging. Vacuum leaks cause lean conditions, which lead to high temps.. and poor vacuum signal or vacuum leak will mess with the idle smoothness.

Check the color of the inside of the tailpipe. If it is real dark black and carbon-like, it could be an indication of a rich condition, but it will most likely be a light grey, which could mean it is running lean. If it was overly rich, it usually masks the pinging, so I'm leaning towards vacuum leak or timing and cap/rotor. Secondarily, fuel delivery, as in injectors, fuel pressure or pump.

Oh, and you may want to check and clean or replace your idle control valve. Passenger rear of the TBI, has a wire harness going to it. It may explain the hunting idle, but not the ping..so once everything is working again, that would be the icing on the cake, but not the cause.

Hope this helps some.

amd103 · April 30, 2002

Great info, thanks a lot!

I just got back from doing some vacuum diagnostics with the vacuum gage. Looks like the engine pulls about 21" at idle (if I remember right that's okay) but as it sputters and hunts the vacuum jumps between 20 and 21.5 or so. It's not clear to me which is driving which...poor vacuum driving poor idle or poor idle driving the jumpy vacuum.

At any rate, I checked the vacuum hoses for leaks and didn't find anything concerning. The PCV valve may need replacing but the engine ran no better with it out of the system and its access port plugged off. The only suprise was when I checked the vacuum line to the charcoal canister. Couldn't read any vacuum to it at all, none from the vacuum port that the line connects to either. Is this proper for a cool engine? Is there a temperature controlled valve that operates vacuum to this line?

So the search for the answer continues, I will check the tail pipe and other indicators as suggested and see where it takes me.

SilveradoWV · May 1, 2002

This brings up a question that I have. Has anyone done that seafoam thing where you put it in a vacuum line???? Just wondering what peoples opinions are on it!

amd103 · May 1, 2002

One more observation I made this evening. I had the air filter off with the engine running, watching the injectors. Both were actuating, but the fuel was puddling on and around the intake ports below them. There was actually fuel collecting and running down the sides of these "ports" into the intakes.

This can't be right can it? If the fuel isn't atomized...well it shouldn't run very well, right? Or is there more going on here than meets the eye? Did my removal of the air filter and housing change the air flow to the point the fuel wasn't able to entrain in the air?

I hope I don't have to buy new injectors!

95Yukon · May 1, 2002

There was vacuum on the fuel canister (charcoal) on my '90 if I remember correctly.. at least after I ran a piece of wire through it then sprayed it with gumout. There is usually a lot of carbon that likes to build up on the inside of those particular throttle bodies. The port on the tbi should have vacuum, but not at the canister, I don't think. It has to go through some soleniod or actuator for it to open up straight to the can, but the initial vacuum feed should be there.

If it ran the same with the pcv blocked off, its probably not that, but wouldn't hurt to spray some gumout in there, too. It should feel like its rattling or pulsing if its working correctly, when it is running.

Check that spark timing and the cap and rotor. Look on the rotor for carbon tracking anywhere and replace them if you didn't, and check spark timing. If all is good spray out the throttle body and do a fuel pressure check, if you feel confident and have the equipment. Other than that, you could start replacing parts like the throttle position sensor/O2 sensor/injectors/fuel pump.. but best to try and pinpoint and eliminate some possibilities first. I ran mine past 200,000 miles before trading it in, and it ran good right up until that day. I always seemed to be replacing sensors on it, though and cleaning the TBI to keep it running good.

The fuel puddling is normal for a TBI. It hits the throttle plates and turns back to liquid somewhat. That is the disadvantage of tbi as opposed to individual port injection. WHen you open the throttle, its not quite as bad, but at idle it looks like it is puddling up (turning from a fine spray to droplets)

95Yukon · May 1, 2002

I tend to think it is low fuel delivery causing it to run lean and ping when hot and under load, and rough idle when cold due to a higher demand for fuel for warmup, which its not getting. You could try loosening the 2 or 3 torx screws holding the throttle position sensor on the passenger side of the TBI (right on the pivot point of the throttle blades) and slightly adjust the top edge towards the front, and retighten it. This will trick the computer into delivering more fuel, and that sensor might just be out of calibration. Theres a little metal tab that rests against a tab on the throttle shaft, which rotates the sensor tab towards the rear when you give it gas. I don't know it that would help at cold startup, but worth a shot. If that helps, I think that sensor is less than 50 bucks. O2 sensor is around $20 usually. Those 2 do most of the fuel control on that setup, assuming the fuel pressure is correct.

amd103 · May 1, 2002

Thanks a lot Yukon, that's a wealth of info. I will give this a shot and keep checking things off as I go. Eventually I've got to find something wrong!

When I fix it, I'll post the solution.