Engine stumbles bad - no codes

[Jpdub]

So this one has me stumped! My GXP with GMPP Stage II tune all but dies when running at lower speeds and yet fails to throw any codes. The problem typically surfaces after stop and go street light traffic and or parking lot expeditions. The engine all but dies, but if allowed to come back to idle it will remain running. When it happens it almost like reaching up and turning off the key. Often, just turning the engine off and restarting on a roll will clear it, but my real fear is getting rear ended when it falls on it's face. The car also goes into a limp mode and will make next to nothing for power and will not make any boost. The problem didn't surface until after the Stage II tune was installed.

I have a ScanGuage II and can monitor a number of engine functions including thrown codes. It has never thrown a code and the only thing that appears to go whacky when this happens is the MAP reading. When the engine stumbles my MAP reading will go from say 12 doing 40 in third to 2.5 and then back to 12 and will continue to bounce around until I do a restart. The problem has in the last few weeks worsened to the point it happens most every day.

I've checked the usuall P0101 code issues such as clamps, intercooler, hoses, etc, but have found no obvious issues. I've even checked the new MAP connections and they appear to have been professionally done using shrinkable butt splices. Again, it has yet to throw a code which I find quite odd considering how bad it runs.

Anyone have any ideas before I trudge off to see the dealership? At least now it is doing it regular enough to reproduce which is why I've refrained from seeing the dealer until now. At his point I'm guessing a faulty Map sensor, but why would it be flakey only at low speeds and never during a hard pull under boost.

[lacajun]

My GXP with GMPP tune has done the same thing. My suggestions are:

1) Tighten all hose clamps - especially the one on the throttle body. Loose clamps will produce this symptom and generally will not produce a code. Replace them with threaded screw clamps as soon as possible. If you continue to push the car, it will show a lean code, maf out of spec, and multiple misfire. Take a ftest drive.

2)Remove MAF sensor and spray with a good electrical contact cleaner. Spray both the electrical contacts and the port open to the airflow on the MAF. Be carefull to not direct a hard spray into the sensors, just wash them and let them dry. Replace the MAF and take a test drive.

3)Be mindfull where you fill up with gas - I have found two stations in my area whose premium will not work in my car. The symptom this causes, is to drop back the timing when on high boost but will operate ok at 7 - 10 psi.

These usually correct the symptom you are describing when no codes show up. This is the method I always follow on turbo powered cars and trucks (retired Master Auto Tech).

[GS Stage 1]

Ditto on the clamps:

The symptoms you are describing are very indicative of a leaky throttle body clamp. You can tighten the OEM clamp and it will feel tight and think there is no problem. However, the throttle body inlet has virtually no lip on it. And the stupid hose will slip down on the back side, against the engine, but it will look completely normal when looking at it on the car.

Mine was causing the same rough idle issues, especially on cold start up. And it finally gave under full boost one day. Sounded like an Elephant farting under the hood. But even then the damn thing didn't come off, it just belched air and of course through the car into limp mode. (huge difference in air pressure being registered between the TMAP and MAP at that point )

On mine, when I pulled the hose off and looked at it, there was an obvious wear pattern on the inside of the hose where it had been sitting way low on the throttle body inlet pipe, on the engine side. It had been sitting so low, that the clamp was half off the inlet pipe.

So I reinstalled, making sure it was shoved all the way up on the back side, and installed a good quality T-Clamp, no problems with that issue since.

[Jpdub]

Thanks all for the suggestions!

Been there done that on the clamps. Checked, double checked and then checked again and I upgraded some time ago to the t-bolt clamps.

I've also tried different gas sources. The first time It happened that was my first thought. I've tried five different brands now of 92, which is the best we can get hereabouts. I also installed Iridium ITV16 plugs to no avail.

There's no indication of any problems with the timing... It's one of the things I can watch on my ScanGuage.

I've not cleaned the MAF. But then there's been no indication of it misbehaving... only the MAP. All indication are that it's on the pressurized side of the intake system.

At this point I'm thinking it's either a problem with my intercooler that I can't see (crack) or the MAP is just flakey. One interesting thing to note is that if I shut it down while it's in limp mode the MAP reading stabilizes out at about 12 when it should be at around the usual 14 (I'm at sea level)???

I've gotten to where I can even tell when it's happening before it affects performance. I can be sitting at a light and the MAP will be sitting at it's usual 4.8 and I'll catch it out of the corner of my eye going to 3.3 or something and sure enough when I try to go at green it will fall on it's face.

[I1ULUZ]

If you take it to the dealership ask them to reflash the ECM, and use the Tech II in the pass though mode.

Mine was loaded using an over the air device and it took a dink, cause phantom codes being set.

My guess in the 20 odd minutes it takes for the flash to load there a lot of chances for the program to take a hit if they are doing it with a wireless setup.

Another thing to do is have the tech ride along with you using the Tech II. He can watch the scanner while you drive so he can see what is going on when it dies.

If you haven't you may want to go back to the stock plugs.

When the key is on and the engine not running do all 3 of the MAP/Baro sensors read the same? Is the IAT reading about ambient air temp if the engine has not been started?

While watching the scanner wiggle the splices on the new TMAPs to see if there is a bad connection.

[sodamninsane]

whats your "MAP" at idle? Are you quoting PSI or kPa? 14 PSI is baro and thats the pre-throttle pressure. Can't be your manifold pressure if you're at idle because the engine would essentially be at WOT...

Have you checked for water in the intercooler? Ingestion of large amounts of water will cause a misfire, but shouldn't hydro lock a running engine.

[Jpdub]

Originally Posted by I1ULUZ While watching the scanner wiggle the splices on the new TMAPs to see if there is a bad connection.

This may be it! Thursday night I pulled the tape off to inspect the splices and then wrapped everything back up. Knock on wood, but it hasn't had a problem since. I also pulled the MAPS and lubed the O-rings. The MAP on the intake manifold seemed awful dry and it didn't feel tight when I pulled it out.

My ScanGauge will only look at a single "MAP" and I'm actually not sure which one it is. The IAT is fine. Reads ambient when I start, runs about 5-10 degrees warmer under sedate driving.

sodamninsane:

Usually the MAP at idle runs about 4.8. When it's having fits it will idle ok but the MAP would bounce around 2-3. With the engine shut off while having fits, the MAP will read around 12 instead of the 14 baro it should be.

[I1ULUZ]

Originally Posted by Jpdub This may be it! Thursday night I pulled the tape off to inspect the splices and then wrapped everything back up. Knock on wood, but it hasn't had a problem since. I also pulled the MAPS and lubed the O-rings. The MAP on the intake manifold seemed awful dry and it didn't feel tight when I pulled it out. My ScanGauge will only look at a single "MAP" and I'm actually not sure which one it is. The IAT is fine. Reads ambient when I start, runs about 5-10 degrees warmer under sedate driving. sodamninsane: Usually the MAP at idle runs about 4.8. When it's having fits it will idle ok but the MAP would bounce around 2-3. With the engine shut off while having fits, the MAP will read around 12 instead of the 14 baro it should be.

I soldered my connectors in before taking the car to the dealership, used heat shrink on each one. Never have been a fan of crimp connectors, they are quick and easy but rather used the tried and true way. Also staggered the joints so I kept a smooth appearance to the run.

One way to tell which MAP you are reading is with the key on and engine not running, have the scanner display the MAP and unplug one at a time to see which one it's reading. Have the scanner display metric units. The one in the charge tube should always read 100 kpa or above, intake should read < 100 kpa at idle and the ECM should say BARO reads 96-104 kpa at sea level. I found in the service manual where it list other altitude values if you need them.

[Jpdub]

Originally Posted by I1ULUZ I soldered my connectors in before taking the car to the dealership, used heat shrink on each one. Never have been a fan of crimp connectors, they are quick and easy but rather used the tried and true way. Also staggered the joints so I kept a smooth appearance to the run.

I had the dealer do the complete install and the finished product was wrapped in electrical tape. I fully expected to find twisted together wires covered in tape, or cheap butt splices, so when I went after the splices the other evening I had in mind doing just as you did. With soldering iron and shrink tubing in hand, I unwwrapped the tape to find the joints were made with the shrink/internal sealant style butt splices. This style of butt connectors are usually weatherproof and more than adaquate. Had they been simple butt splices I would have proceded to go the solder route, but I left them as is and rewrapped them inside some split loom tubing.

Still no re-occurance of the stumble so it's looking like either the lubing of the O-rings or the moving around of the splices was the fix. Fixed or not, it looks like I'll be replaceing the existing splices with solder and shrink tubing just for the assurance it does not reappear.

[sodamninsane]

sounds like you're getting a voltage drop out of the MAP sensor. I'd go after those splices only, then wait and see if the problem happens again. MAP going down during a stall is pretty unlikely, usually it goes up because the engine misses and the speed control opens the throttle to try and build more torque.

[Jpdub]

Originally Posted by sodamninsane sounds like you're getting a voltage drop out of the MAP sensor. I'd go after those splices only, then wait and see if the problem happens again. MAP going down during a stall is pretty unlikely, usually it goes up because the engine misses and the speed control opens the throttle to try and build more torque.

I agree! It's wierd! If the engine misses, the MAP should increase. Same is true if the throttle body tried to compensate... MAP should increase. Same with when I shut her down while having issues. No reason I shouldn't see ambient baro pressure rather than a couple of lbs less than baro, but that's what was happening. When it's whacked out I was seeing anywhere from 2-10 lbs less pressure than what I should have under the circumstances. So much for being "absolute".

[GS Stage 1]

Even if your clamps are all changed and good, I would still be suspicious of a leak in the charge system somewhere. (a leak will cause inverse reactions, lower pressure than anticipated under boost and higher pressure than anticipated under non boost):

Also, I found out the hard way that a "mushroomed" IC will cause the dreaded P0101 air charge code. The running theory was that the different tube sizes were causing variable air velocities which in turn freaked out the TMAP and MAP. Here is a link to my problem child thread: GMPP Tune and P0101 code. You will notice that I have had just about everything go wrong with my car that is possible. But this thread was begun to deal with the air charge code issues and there is a lot of good information in here.

And I also remember one forum member (I think he was on the Sky side) that found a "crack" in his IC that he could not see until he started taking parts off. He was having the same dreaded P0101 code and started investigating further after reading about my issues.

["Maine"-iac]

I had a similar problem when I did my supercharger install. It turned out to be leaky intake tubes and the MAP sensor. After tightening the hose clamps around the tubes the problem still happened. My MAP sensor kept popping out of the hole causing a pressure leak. The psi readings were all over the place and it had erratic idle and throttle response. I swapped the MAP around to fit in the hole better and it fixed the problem. Not sure if they removed your MAP during the tune but just in case make sure it is seated tightly in the hole. You may also want to check to see if any sort of debris got stuck in the MAP sensor itself.

Off hand it sounds like what GS Stage 1 is talking about with a faulty intercooler. There has been a history of the stock IC not being able to hold the additional pressure that the system puts out and causes cracks along seams or blowouts elsewhere on the unit.

[I1ULUZ]

Need to figure out which is the cause and then the effect.

HPT has a multi line scanner that can display various parameters at the same time. I would log RPM, MAPs, TPS, WBO2 and MAF to see which parameter starts the chain reaction and how the ECM reacts.

My guess is the TMAP in the intake is the main one that controls fueling and timing, it being dry and ill fitting could cause issues. Was the DIC reading the normal boost readings at WOT?

I think I would have the engine running and move the harness around to see if you can make act up again just to make sure it does not leave you sitting by the road in the future.

[Jpdub]

No issues again today, and that after doing a bunch of around town driving today. So far the cause and effect seem to be related to untaping, inspecting, and re-covering the splices to the intake manifold MAP and pulling the intake manifold MAP out and lubing the O-ring. After seeing that the splices were not an appearant problem my money is on the map O-ring not properly sealing.

After major issues last Thursday and working on it that evening, that's all I've done, and I've not had the problem since.