Hi Guys,

I am helping my Nephew to restore his IROC-Z back to near-factory status.

Car Details:

1987 IROC-Z, 5.0L TPI, 5-Speed, Federal-Emissions, 16" Wheels.

165 ECU, $32B Mask, MAF Based.

Full Emission Controls.

Car Modifications:

Edelbrock Headers.

3" Flowmaster Exhaust with 80 Series Cross-Flow Muffler.

Magnaflow CAT.

AFPR.

19 lbs/hr Bosch III Injectors.

AFS75 Heated NB02.

Tayor Plug Wires and MSD High Voltage Coil.

The Problem:

After the ECU enters Closed-Loop, the idle BLM approaches 150. The off-idle BLM values are close to 128.

The exhaust smells rich.

There may be a slight miss at idle.

Trouble-Shooting to Date:

Air Pump Belt Removed. No AIR injected in the header AIR Ports.

The Throttle Body vacuum port has been plugged. This removes the EGR and CC vacuum connections.

A Plenum vacuum gauge has been installed where the Heater/Vaccum-bulb line is normally connected.

Fuel Pressure is ~ 3 BAR or 43 PSI.

Plenum Vacuum @ 800 RPM is 18-20 mm Hg.

MAF is ok. At idle around 6 grms/sec.

Coolant Temperature Sensor is ok.

TPS is ok. However, the idle IAC value count is 70. Very high!

Thoughts:

With 18-20 mm Hg Plenum vacuum at idle, can an intake vacuum leak exist?

How would I investigate a potential exhaust leak around the NBO2?

How much of a miss at idle would cause a false-lean condition?

I will investigate if the miss is the result of an ignition problem or a fueling problem.

The hope is the miss is not a valve based problem. The engine was re-built before my Nephew bought the car.

Any thoughts on how to trouble-shoot this problem would be greatly appreciated.

Best Regards,

Bruce

 

Sounds like you already have a good diagnostic plan and good understanding of the functioning of the various systems.

I'll just add that a modest vacuum leak can be covered up by the computer at idle, especially if it's not very close to a single individual cylinder. Yes, you can still pull plenty of manifold vacuum even with a vacuum leak, it's just that some of the air is coming in unmetered by the MAF and therefore not being accounted for until the O2 comes on line and the ECM realizes you're running lean.

And vacuum leaks have their largest effect at idle, in most cases. A small amount of air coming in unmetered at 1/4 throttle or more is just a tiny percentage of the overall volume of air going into the engine. But at curb idle that same vacuum leak is a much larger percentage of the total air going into the engine.

Don't forget the obvious stuff like PCV valve and power brake booster which can give a "hidden" vacuum leak.

If you suspect valvetrain problems the first test I would do is a compression test on all 8. If something is whacked you see a cylinder (or two) that is noticably higher or lower than the others.

 

Hi Damon,

Thank you for the response. It is very appreciated.

Yes, my Nephew and I plugged the PCV port at the Base-Plate Port. I forgot to mention this in the original part of the thread.

No. We have not yet removed the Power-Brake Booster line and plugged the port.

No, we have not yet run a compression test. Given a recently rebuilt 5.0L, what would be a decent cranking mode PSI for the cylinders?

Could the AFPR have a vacuum leak? We did not pull this port either.

What would be the best way to search for a potential vacuum leak?

After searching many sources, the most common method appears to be the Carb-Cleaner method.

Does Carb-Cleaner attack gasket material?

If I use Carb-Cleaner and I find no vacuum leaks, will I have to change the gaskets due to reduced service life?

Are there better vacuum-leak detection methods?

Best Regards,

Bruce

 

I prefer to use a propane torch or starting fluid to look for vacuum leaks.

For hard to find exhaust leaks you can try looking in total darkness while revving the engine up and down.

Keep in mind that the EGR valve itself may have a small leak.

As you know the BLM is a self-correcting system, but is not infallible. A weak, clogged, or otherwise malfunctioning injector on the band with the O2 sensor can cause the system to over-correct.

There may also be a small vacuum leak around an injector, to include the cold-start injector if you have one.

 

I have the same basic engine, however mine is modified. Still, I have had to tweek MAF Table #1 to get the idle BLM's in line.

FYI, My engine compression reads 173 psi +/- 3%

 

Thanks 305SBC and mnorton,

When testing for vacuum-leaks with a combustible gas, should I remove the IAC connector?

Just wondering if the ECM Idle control logic would act fast enough to limit the RPM change that results from the leak-test?

The injectors are re-built. Can I test the injectors when installed in the car? Maybe look at a Fuel-Pressure drop as a function of the number of injection cycles?

The EGR valve is new. I wonder if the valve quality is low?

What are the typical areas where vacuum leaks occur? I will focus on testing these areas.

Best Regards,

Bruce

 

vacuum leaks are a son of a gun to
find when they are very small

 

What are your 02 readings on the scan tool? Are you throwing a code 44 by any chance?

A common problem is a bad wire on the 02 circuit or an internal short in the ECM on the 02 circuit. This will cause a rich condition and high BLM's at or around 150 or more.

Skin back the purple 02 wire in the harness right after the 02 plug and check what your readings are coming out of the 02. Then check that same wire's reading right before it goes into the ECM. If the readings are good at the begining of the wire and end of the wire. Then you know the wire is good. But if it is inconistent from the front of the wire to point right before it goes into the ECM the wire is bad.

If the reading are same front and back on the wire but do not match what is showing on the ECM. The ECM is bad.

If you do not have a service manual to look up what wire position for the 02 wire on the harness before it goes into the ECM let me know and I will look it up for you. But is should be position D7 and the ground for the circuit is a tan wire in postion D6.

If you have trouble running my testing I can PM you my number and help you out over the phone.

Good 02 readings should be varying rapidly between 300 mv to 700 mv.

Also if you disconnect the 02 sensor plug on the car with it idling what reading do you get on the scan tool? should be between 350mv to 550mv. If not we got a short in the ECM or wire.

 

The IAC (Idle Air Control) Valve controls the "Air Leak" and could need to be replaced. It just a Stepper Motor... Probably about $50 from GM. Easy to replace.

 

Hi Guys,

Thank you for the suggestions.

I have been thinking about this problem for a while and looking at the TunerPro RT datalogs.

Do I have an intake system vacuum leak or an exhaust system leak that is close to the drivers side NBO2?

Here are the symptoms:

• Black spark Plug Insulators.
• Very Rich Smelling Exhaust.
• Idle INT=128.
• Idle BLM=145.
• Plenum Vacuum ~ 20 mm Hg.
• Fuel Pressure ~ 3 Bar.

If I had an intake vacuum leak, i would think that the fuel trim would bring the overall AFR back to 14.7:1. The "extra" air (entered after the MAF) would lean out the mixture until the fuel trim could compensate. The Spark Plugs should look ok.

If I had an exhaust system leak, the mixture within the combustion chamber should be ok (correct MAF and Injector Constant). However, the NBO2 reads lean due to excess O2 that is sucked into the exhaust system (near NBO2). This would cause the ECM to trim to mixture to a fairly rich state. the result is a rich smelling exhaust and possibly lousy looking spark plugs.

Make sense?

If so, shall I search for the exhaust leak with "Seafoam" or the "rag up the exhaust pipe" method?

Any other known good exhaust leak searching methods?

Thanks again for the help.

Best Regards,

Bruce

 

Black insulators on the outside of the engine or the tips?
Is the ECM staying in closed-loop at idle?

Misfires at idle can cause misleading O2 readings.

If you eliminated the possibility of injector problems (swap sides with injectors), and a possible leaking FPR, then I'd say chances are high for either an exhaust leak, or a malfunctioning O2.
Some exhaust systems have trouble keeping the O2 hot enough at an idle. I've seen this on a few factory stock vehicles. Did you try another O2 yet?
I have personally seen many part-store replacement O2 sensors not work right, new out of the box. It seemed to be brand dependent. I'm thinking it was the Bosh sensors, but I'm not sure I remember correctly.

I check for exhaust leaks by using my fingers (yes I know dangerous), looking for soot marks, a spray-bottle with thick soapy water, or visually in complete darkness while revving the engine.

Last suggestion, I would verify the AFR with a wide-band before proceeding. The O2 is seeing a lean condition and the ECM is trying to correct it. There is a chance it could be right.

Edit: I forgot to ask if you've eliminated the AIR system on the manifolds completely or if the tubing and check-valves are still in place? I know the tubes and valves can be a source of exhaust leaks. The pulsing of the exhaust can pull fresh air in through the check-valves.

 

come to any conclusions?