I currently have a 92 c1500 with a tbi 350 that seems to stumble at idle either when in gear or in park idle but only in closed loop. I've been troubleshooting this for about 3 weeks and not getting anywhere. The truck was originally a 305 but I had just swapped in a GMPP 350 with vortec head. I'm using the whole tbi system including the EBL Flash II. I've got all new sensors including the knock sensor and esc which I read are specific to engine size. It had been running good up until the one day it backfired through the TB and had to idle all the way home. Found the distributer had moved, reset that. With it idling in the garage, once it stumbles enough to stall, it will get code 23 which I read was IAT even though the truck doesn't have it. Using tunerpro with the EBL and the parameter for coolant and IAT is set at 100 all the way across which I think means only the coolant temp is affecting it. I'm wondering if anyone knows what else can be checked for that IAT code 23, if it can be related to anything else. The other thing I found recently is when I was testing continuity on the ignition coil everything was OK except test #3 which was infinite where it says there should be resistance. Both old and new coil get the same thing. The pickup coil checks out, brand new distributer and replaced the ICM and used the thermal paste. The coolant temp seems to be working properly aswell. If anyone has any direction they can think of would be great. I also have an 18psi spring for fuel pressure and when testing get right around 18-19 psi along with new ac Delco injectors and a new stock vortec fuel pump. I have tried checking for vacuum leaks but only had throttle body cleaner, not sure if its any different than carb cleaner but didn't find any obvious leaks. The transmission is a 4l60/700r4 that was rebuilt, I just replaced the vss since it was old with no change aswell.

Any help is appreciated, feel like I might be overlooking something simple.

Thanks.

Also I'm new to posting on forums. So if this is in the wrong spot let me know and I'll figure out how to move it.

 

I'd suspect a vacuum leak. Smoke check is the most effective way. It is a bit more difficult to do with a tbi or carb. What I did was taped up an old mounting gasket and bolted it underneath the carb.

 

Ok I will look into trying the smoke test this weekend. Low vacuum would make sense. The cam thats in seems fairly mild but the map is at 50kPa at just 750rpm. If I try and lower the idle anymore the map goes over 60, so would that lead even further to a vacuum leak. The engine is the sp350/357 and the cam specs are:
The duration at .050" tappet lift (intake/exhaust) is 215/223 and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 473/473.
Does those sort of map readings make sense for that cam? To me they seem high.

 

The tb will be divorced from the smoke tract, so anything that's connected to it (I don't recall how many lines go it it) you can test with a hand held vacuum pump. A slightly sticking egr can cause a similar problem.
I'd also look at tb passages and tb blade closure.
If it all checks out then it's likely somewhere in the tune.

 

If it runs good in open-loop, that rules out most of the basics--compression, idle fuel pressure, ignition except for electronic advance, vacuum leaks, leaking EGR. All that stuff would affect open- or closed-loop operation with the exception of the open-loop mixture being slightly richer, and that might cover-up other problems.

What happens when the engine is fully-warm, has run in closed-loop for awhile, and then you disconnect the O2 sensor to force open-loop operation? Still misfire? Or idles good?

The injectors are new, but what injector size? Wrong injectors would be more likely to affect higher-rpm/higher load operation than at idle, but it's worth telling us about.

What does the '92 service manual say about Code 23; and the conditions needed to set that code?

Incorrect range on the meter? I'd expect you'd need to switch range between the ~half-ohm of the primary, and the ~15,000 ohms of the secondary.

Connect a scan tool, verify EVERY sensor and computer output. See what the sensor data stream is showing when the engine stumbles.

Some folks use aerosol brake cleaner, which can be very dangerous. Far as I've seen, throttle-body cleaner is as flammable as carb cleaner--should work fine. What happens when you spray the throttle-body cleaner into the running engine?

 

In open loop sometimes the idle is affected just not as bad, like idle will move around a bit but becomes way more noticable when in gear or when in open loop, sorry I should have made that more clear in the opening post. The egr is also gone and blocked off with a gasket and has been taken out of the tune in the computer.

I haven't tried that yet but I will do that and see what happens.

The injectors are 61 #/hr which replaced my 305 ones. I've replaced the injectors since the issue has started aswell.

The service manual for 92 pickups doesn't actually have code 23 in it just 22 and 24. I don't think the trucks came with an IAT, so I don't know if the EBl Flash is picking something up?

I'll try that as well with a wider range meter.

I've been watching everything on the "whats up" from the EBL and haven't noticed anything out of the ordinary yet. the engine runs ok when the idle drops from 750 to around 300-400 then jumps up to 1000 before it settles again. It seems like everything starts jumping around once the idle has dropped. watching the spark advance on the "whats up" shows it moves around a bit, like it will jump from 14 to 24. I have the SA park idle set at 22° .

I'll stick with the TB cleaner then. when you say into the running engine, do you mean through the throttle body to check the throttle plates?

 

I bet it's a false code.

Wild Guess: You don't need a "wider range meter", you just need to flip a switch or push a button on the meter you have, to get it into a higher range.

What is the Idle Air Control doing? Fuel trims?

Yes, through the throttle body, to see how the engine responds. I don't intend this to be a check of the throttle plates.

If the engine doesn't respond to the spray through the throttle body, it's not going to respond to the spray through a vacuum leak.

 

Ya, I was using a meter that didn't have any range adjustment. Found my other one and it tested ok.


The IAC does move around, can actually here it letting more or less air in. I'm guessing from trying to catch the idle from stalling.

Tried spraying the TB cleaner directly into the throttle plates and the engine does bog down, so started searching again for vacuum leaks and eventually got it bog when spraying where I have the brake booster into the intake. It would only do it in a very specific angle and spot on the fitting. added sealer and going to let it set overnight and try it in the morning to see it it makes any difference. Also was watching a vacuum gauge while doing it and it consistently stayed around 13-15" Hg and would drop when the engine bogs down. does 13-15 sound normal for this engine. I read stock is around 20", not sure if a mild cam would bring it down that much.

 

I would have used a clamp, not "sealer". Or are you talking about a pipe-thread metal fitting?

13--15 inches of vacuum seems low to me.

Was the "mild" cam degreed when installed, or just "dot-to-dot"? What is the cranking compression?

Does the spark advance work properly? Initial advance correct?

 

Put new sealer on the threads, had a hose clamp on the barbed end. It's an engine from GM so I never installed or adjusted the cam. Initial advance matches the computer and seems to advance properly. Now that the brake booster is sealed properly it runs way better and how it should.

Only thing now is I pulled it out the garage and it went ok, half hour later went to take it for a test drive and could hear a whining/grinding noise that I can only explain like a gear drive timing system. I took the drive shaft out and its ok (the U-joints were done recently and all move freely, the diff makes no noise. Ran the engine and put trans in gear and noise immediately starts. From what im reading it could either be the trans filter system or possibly torque converter. The Transmission was rebuilt by a shop with a new torque converter, but I did install it myself so anything is possible now. The fluid seems ok the level was a bit high from when I filled it, not sure if that can cause issues aswell. I can't call the shop for a couple days until when they open again. wondering if anyone has had a similar issue?

 

When you bolted the converter to the flexplate, did you have to pull the converter forward? There should have been a gap between converter and flexplate. If there wasn't, good chance the trans pump is now ruined.

 

Kinda jumped the gun asking about the transmission. Forgot that I had changed the VSS while the truck was down. Pulled it out and found it was contacting the reluctor gear. That's all good now. It seems to run better overall, I think its just tuning left to get the idle a bit better. Once it goes into closed loop it bounces between 600-800 rpm both in park and in gear, where as before it would drop below 500 and stall with that vacuum leak I had found. I'm completely new to engine tuning and its definitely a learning curve.

 

This graphic got posted on another forum. I took a closer look at it--saw something I missed entirely when it was posted here.

Tests 1 and 3 are valid and useful. Test #2 is crazy. Now, anything is possible but I've never seen or heard of those two "A" terminals NOT being connected inside the coil. One of them is the battery-voltage input power for the coil. The other is the connector that supplies that same battery voltage power to the ignition module. It's a pass-through for the primary power.

In the same fashion, the two "B" terminals are also connected inside. They're both connected to the other end of the primary windings. One terminal leads to the ignition module, so the module can ground and un-ground the primary windings, the other leads to a tachometer if the vehicle is so equipped. You could test for continuity from "B" to "B", but it's a near-useless test. If there's no continuity to the Tach terminal, the tach doesn't work. If there's no continuity to the one that leads to the module, the engine doesn't run, and so the tach doesn't work. But I've never seen that problem either.

The test not shown on that diagram, that would actually be useful is to check resistance from either "A" terminal to either "B" terminal. Doesn't matter which one, as long as you test from "A" to "B". That would show you the primary winding resistance, which for HEI coils is typically around 0.5 to 0.7 ohms. A coil for a ballast-resistor style ignition--points or the original Chrysler electronic--would be 1.5--2.5 or so.

Do keep in mind that when testing very low resistance items--like the half-ohm primary resistance--that you'll need to "zero" the ohmmeter by putting the free ends of the test leads together, so that before you measure the component, you've accounted for the resistance of the test leads. My Fluke 88 multimeter test leads have about 0.2 ohms all by themselves, so when testing a component like the primary windings of an ignition coil, the reading I get is more like 0.7--0.9 instead of the correct 0.5--0.7. I have to subtract the test-lead resistance to get the true reading. (Or I can remember to put the leads together and press the "Zero" button.)