LT1 running rich - not sure where to go next?

[SenselessSteve]

Hi Jim. I didn't get to work at the boat on Saturday as planned but finished up in work early today to get to the garage.

I run the checks you suggested and I've discovered that the return voltage (green wire) at the MAP sensor is reading between 2.6 and 2.9 VDC at idle. The plot thickens.

The voltages from the plug all check good when the ignition is on (but not started).

I'm waiting on the MAP sensor arriving that Mike got me a part number for but perhaps there is more to it than this. In your experience, is it common for the MAP to fail and deliver a high reading or is this scenario synonymous with vacuum leak?

I've encountered scenarios were an ECM will substitute a fixed voltage if the MAP fails but perhaps I can rule this out given the variation between 2.6 and 2.9 reading??

Thanks in advance for any input. It's greatly appreciated.

Stevie

[JimN]

Hi Jim. I didn't get to work at the boat on Saturday as planned but finished up in work early today to get to the garage.

I run the checks you suggested and I've discovered that the return voltage (green wire) at the MAP sensor is reading between 2.6 and 2.9 VDC at idle. The plot thickens.

The voltages from the plug all check good when the ignition is on (but not started).

I'm waiting on the MAP sensor arriving that Mike got me a part number for but perhaps there is more to it than this. In your experience, is it common for the MAP to fail and deliver a high reading or is this scenario synonymous with vacuum leak?

I've encountered scenarios were an ECM will substitute a fixed voltage if the MAP fails but perhaps I can rule this out given the variation between 2.6 and 2.9 reading??

Thanks in advance for any input. It's greatly appreciated.

Stevie

I don't remember the default for defective MAP, but the TPS defaults to 12°. Make sure that's operating correctly, too. I don't know if they would run it this rich if the MAP were to fail, though.

Idle, WRT functions of the ECM is <2° throttle position.

Look at the IAC after the engine is turned off. Start the engine and watch the IAC- if the pintle closes and the idle speed is high, it could be a vacuum leak that can't be countered by the IAC and a leaky diaphragm in the MAP sensor could explain this. I think I mentioned applying vacuum to the MAP sensor- if it holds this vacuum, it's not leaking but if it loses vacuum, it's leaking. Also, make sure the tube that connects the MAP sensor to the ECM fits well and isn't leaking. It would tend to leak at the ends.

[SenselessSteve]

Thanks Jim. I read another thread you had helped with and I tried the paperclip test. It flashed codes 14 and 21.

I'll try to google what they are now

[SenselessSteve]

Also - there doesn't appear to be a tube that connects my MAP to the ECM and no place for one to go?? The plot thickens further.

Keep a suitcase packed Jim - you might get that trip to Ireland :-) thanks for your help

[JimN]

Thanks Jim. I read another thread you had helped with and I tried the paperclip test. It flashed codes 14 and 21.

I'll try to google what they are now

21 is TPS High, 33 and 34 are MAP sensor and 14 is ECT High Temperature.

The MAP sensor attaches direct on the intake- look at this link for a rectangle on top with two screws, along the far edge of the flat surface. THAT's where your MAP sensor is supposed to mount and it doesn't use a tube. If the tube isn't connected and has no vacuum applied, the sensor is only reading barometric pressure and thinks the engine is running at WOT (Wide Open Throttle), causing a very rich condition.

How high the TPS reading is remains to be seen- open the throttle a bit and see if it idles- if it does, it's possible that a TPS from a regular engine was installed and that doesn't work. The LT-1 TPS is reverse and if the regular one is used, the ECM thinks the throttle is WOT and won't run. WOT during crank is how you would clear a flooded engine and no fuel is delivered at that time.

Ordinarily, 14 and 21 would result in a very lean fuel mixture (high temperature needs less fuel and if it runs lumpy, it may be in RPM Reduction, which others call 'Limp Mode', incorrectly. 21 may result in a default setting of 12%, which causes it to run, but not at its best. If that was the only code and situation, acceleration would be sluggish and it probably wouldn't achieve top speed.

[SenselessSteve]

I tested the voltage on the TPS (engine off). It ranges from about 0.7 to 4.2 at WOT.

I removed the MAP sensor to check the rubber gasket where the MAP vacuum pipe inserts at top of the intake manifold was was making a good seal - all appears fine.

I noticed that the plate with 6 screws on top of the throttle body appears to have a very distorted gasket underneath. I opened it up to reveal a very distorted rubber gasket that seems to be heat damaged. I'll need to replace this anyway as it's a mess. Have you ever had this cause trouble with mixture?

Thanks again

[JimN]

I tested the voltage on the TPS (engine off). It ranges from about 0.7 to 4.2 at WOT.

I removed the MAP sensor to check the rubber gasket where the MAP vacuum pipe inserts at top of the intake manifold was was making a good seal - all appears fine.

I noticed that the plate with 6 screws on top of the throttle body appears to have a very distorted gasket underneath. I opened it up to reveal a very distorted rubber gasket that seems to be heat damaged. I'll need to replace this anyway as it's a mess. Have you ever had this cause trouble with mixture?

Thanks again

I never needed to remove the cover, so, no.

.7VDC-4.2VDC is fine but the code still bothers me. Disconnect the battery for a half hour, reconnect and check for codes again. If you see none, start it and see if it's any better.

Did you disconnect/reconnect any of the sensors while the key was on or the engine running? That can cause a code situation.

[SenselessSteve]

Hi Jim. I disconnected the battery for an hour while I had dinner. Came back and reconnected (all sensors in place).

I started the boat and let it idle while I measured the voltages.

TPS at idle was around 0.72 and increased steadily when I applied throttle.

Checked MAP. Steady 5V at the grey control but when I measured the green return it was bouncing around 1.6 (at idle). Applying throttle made the Voltage decrease (to around 0.9V at 3500rpm)

Still strong smell of gas and I could actually see it slick in the water gathering on the ground below the flaps.

Tried the paperclip exercise and again I had codes 14 and 21.

Also felt that the riser on the port side was a lot cooler in comparison to other.

[JimN]

Hi Jim. I disconnected the battery for an hour while I had dinner. Came back and reconnected (all sensors in place).

I started the boat and let it idle while I measured the voltages.

TPS at idle was around 0.72 and increased steadily when I applied throttle.

Checked MAP. Steady 5V at the grey control but when I measured the green return it was bouncing around 1.6 (at idle). Applying throttle made the Voltage decrease (to around 0.9V at 3500rpm)

Still strong smell of gas and I could actually see it slick in the water gathering on the ground below the flaps.

Tried the paperclip exercise and again I had codes 14 and 21.

Also felt that the riser on the port side was a lot cooler in comparison to other.

If one riser was warmer, it's fairly normal at idle, but at higher RPM, it indicates a problem. If it was on the trailer, don't run it at high RPM- the water supply usually can't keep up with the raw water pump.

What is the resistance of the ECT? (the one with one yellow and one black wire). If it's 20°C/68°F, the resistance should be about 3520 Ohms. If it's significantly lower, it's bad. If it's higher, it would indicate low temperature, which would cause a 15 code.

[SenselessSteve]

I went to check the resistance (boat still Luke warm from earlier testing) and it was reading 1240

[JimN]

I went to check the resistance (boat still Luke warm from earlier testing) and it was reading 1240

Here's a list of resistance values-

http://www.hptuners.com/forum/showthread.php?11265-ECT-resistance-values

Measure it again after it cools completely.

Next time, feel the manifold next to the ECT- if it's noticeably hot, measure the resistance and compare it to the table in the link. You'll need to scroll down a bit, but copy it to a Word (or other) document for future reference.

[SenselessSteve]

Thanks Jim. I measured the resistance as the engine ran from cold at idle.

It started off at around 3600 Ohms and fell steadily to around 1000 Ohms before I shut it off.

I ran for codes and again got 14 and 21.

Very strange indeed. I've also head the fuel pump starting to squeal quite a bit as it pressurises so I'm going to check fuel pressure again once I get my pressure tester back off a local borrower (damn I hate lending out tools)

Thanks again. The show goes on

[jlarkin.41]

Does it idle poorly, or is it relatively smooth? Assuming the exhaust smells of unburned gas, try this- unplug the MAP sensor tube and listen for any changes in the idle speed. If it changes, that's good. If not, check for vacuum at the end. If it has none, find out why.

If you have a multi-meter that will measure voltage and resistance, insert stick pins or T-pins into the wire side of the silicone seal on the MAP sensor wires, sliding past the seal and making contact with the terminals. Bend them so they can't touch each other. Use test leads with alligator clips and attach them to the pins.

Measure the resistance between the MAP sensor ground and ECT ground- IIRC, they share the same ground point and wire to the block. Measure the resistance to ground from the MAP sensor plug's ground terminal and also from the ECT's ground terminal- if you see more than about .1 Ohms, it's too high. If this checks OK, clip the ground lead of the meter onto a good ground on the engine block.

On the MAP sensor plug, Gray is control, Lt Green is the return voltage wire and Black is ground. The Gray should have ~5VDC at all times and the Lt Green should have a range of just less than 1VDC-4.5VDC. At idle, you should see about 1VDC on the Lt Green wire. If it's higher, or the Gray wire doesn't have ~5VDC on it, there's a problem. If the Lt Green wire shows higher voltage, it's telling the ECM that the engine is not at idle- low vacuum means the throttle is open, which requires more fuel. Low vacuum=high voltage=open throttle and since the MAP senses the vacuum below the throttle plate, this is critical.

With the key ON/engine OFF, measure the reference voltage on the Gray wire, using the pin in the ground position and the Lt Green wire should have high return voltage because there's no vacuum. Connect the vacuum gauge to the MAP sensor and pump it a bit- you should see the voltage decrease as the vacuum increases. You may not see much needle deflection on the gauge. If you never see needle deflection or voltage change, it's possible that the sensor is bad. If you have a GM parts provider, you could ask for the large, 1 Atmosphere GM MAP sensor. They also have a small one- this is used on the LTR and inserts in the manifold, so it won't work. The TBI and LT-1 used the same MAP sensor.

Hey JimN- I am having what seems to be a temperature related rough idle and stall condition on my LT1 (when hot and after heat soaking). I have a post about it here: https://teamtalk.mastercraft.com/sho...93#post1527693

Anyways I was following your test of the MAP and ECT common ground and came up with .2 ohms. Any thoughts here? You say above it should be below .1 ohms, just wondering what would cause the resistance, corrosion possibly? I don’t see any. Don’t know if this is possibly related to my issue or not.