Dear Roy,
I own a 1995 four-wheel-drive Chevy Tahoe with a 5.7L TBI engine. As the second owner of this SUV, I began to tackle the problem by first pulling any useful info from the data stream with my M2500 scanner. No codes whatsoever.
However, when warming the vehicle at idle, I caught a glimpse as it went into close loop at 1,500 rpm: The EGR monitor suddenly went to 100 percent with the EGR solenoid energized to
"On," followed by a surge.
I let go of the accelerator; it almost stumbled but regained its normal rpm. When driving the vehicle, it would act as if it had a bad TCC solenoid. But when accelerating from the engine compartment, the EGR - as seen from its lower vent - would move 1/4 of an inch at 1,500 rpm, let go for a few milliseconds and bounce back fully open.
I tested the EGR solenoid resistance, and it checked out OK. There was no carbon obstructing the pintle or EGR passages. I also installed my EGR solenoid on a friend's 1995 Tahoe, and it worked.
I made a vacuum test on the EGR, and it moved freely with a minimal vacuum loss. To document it, I took EGR vacuum drop readings, comparing them to my friend's EGR on his Tahoe. At the end, the readings showed both EGRs holding vacuum, the needle slowly dropping from 18 inches to zero in two minutes, and his Tahoe is working fine.
What have I overlooked? Any help you could give me would be greatly appreciated.
Thank you,
Othon, M. Basto, via e-mail
Dear Mr. Basto,
GM used a negative backpressure or port EGR on some vehicles over 8,500 pounds and a linear EGR on those under that weight. So first, we must determine which EGR system is installed on your Tahoe.
The negative backpressure and port use a separate solenoid and can be identified by a P = Positive backpressure or N = Negative backpressure stamped on the EGR valve housing. The linear valve is self-contained and the EGR pintle is motor-driven with a position sensor. As you refer to a vacuum test of the EGR, I am going to assume that your engine is not equipped with the latter.
The negative backpressure EGR has an internal air bleed that is held closed with a small spring when there is no backpressure. When the engine is at idle with vacuum and backpressure, the small internal air bleed is open and vacuum cannot open the EGR.
With the engine running, this system may have vacuum applied to it from the solenoid at idle, but the valve should remain closed.
In order to test the operation, turn the engine off. Apply 18 in/Hg vacuum with hand pump to open the EGR. Start the engine and immediately observe the EGR. The valve should close and the vacuum should drop. If not, we have clogged passages or a defective EGR valve.
The Chromed No-StartDear Roy,
A customer damaged the air flow meter on a 2003 Nissan 350Z (VIN JN1AZ34D73T118402) while installing chrome accessories. The vehicle has a 3.5 VQ35DE engine and manual transmission.
We repaired the broken wire to meter, but the vehicle would not start. We scanned with the Modis, but the vehicle would not communicate. We ordered and installed an ECM, but still no communication.
Please help.
Lynne Walker, Owner
Pete's Auto Service of Denbigh Inc., Newport News, VA
Dear Lynne,
OK, we will try to help you. First, what wire was actually damaged? Did you check the fuses and fusible links?
Assuming all this started at the mass airflow, let's check here first. Do we have battery power to the R/W wire at the MAF? This is supplied power from the ECM relay to many engine management components including the PCM. If power is not present, remove the ECM relay from the under hood power distribution box. With the key on, test for power at two of the terminals. If power is not present, check fuse #82 (15A) for continuity. If all these test well, go back to the MAF and test the B/W wire for a good ground. This is a computer-generated ground that affects multiple sensors.
Hope this gets you going in the right direction.
Hard-Braking TaurusDear Roy,
We have a 1992 Ford Taurus, VIN 1FACP5U9NG211790, with a 3.0L engine and AOD transmission. When it first came in, the car had a high effort/hard brake pedal accompanied with an idle that would
"hunt" up and down for about the first five minutes after start-up and then normalize.
We jumped to the inaccurate diagnosis of a bad brake booster. Its replacement made no difference; there was so little booster assist it made the car unsafe to drive.
We subsequently examined the vacuum plumbing, which did not appear to be tampered with, and we checked the timing.
By trial-and-error, we found that removing and plugging the hose in the intake duct restored normal brake feel but resulted in a no-idle condition. We are seeking your help with this problem.
Tony Comunale, Owner
Automotive Endeavors, Churchville, NY
Dear Mr. Comunale,
These vehicles had a problem with carbon clogging the vacuum port to the booster. This problem required a thorough cleaning at the port.
However, in this instance, you were able to restore vacuum to the booster by removing and plugging the crankcase fresh air hose. Ford calls it the PCV system closure tube.
My suspicion is that your PCV valve is either stuck open or destroyed, allowing false air and reducing vacuum to the booster.
No PressureDear Roy,
The vehicle is a 2001 Chevrolet Silverado with a 5.3 Vortec engine and 700R4 transmission (VIN 2GCEC19T011295591). While turning the steering wheel completely right or left, the brake pedal loses pressure and goes to floor.
We replaced the master cylinder, but it did not correct the problem.
Please help.
Johnny De Los Santos, Garage Technician
Total Autocare, San Antonio, TX
Dear Johnny,
Carefully check the banjo bolts at both front and rear brake calipers. These bolts retain the brake hose to the calipers with a copper washer on each side of the fitting. These bolts have been known to loosen and leak when the hose is stressed by turning the wheel or from certain off-road operations.
