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Instead of Chasing GhostsPosted 5/7/2002By Jeff Bach
Hesitation. Flat spot. Just won't go. Bogging. Momentary loss of power. Fails to respond properly when accelerating. Feels like mush. Wouldn't pull the hat off your head. These, and a few others not worth mentioning, are phrases I hear regularly from customers describing cars that have the feeling that ... well ... you get the point. I am currently working on an '82 coupe that my customer says “feels like it needs a tuneup.” Now, there's a jumping off point. A little needed maintenance before we begin attacking the “problem.” This one feels OK when you first start off cold, but once the engine warms up it picks up a “flat spot.” By flat spot I mean it's bad enough at times that the engine gets so close to stalling that the oil pressure light will flicker before it finally starts slowly picking up speed. With the throttle held down, the engine slowly picks up to normal power. Because of my experience with this model, too many possibilities came to mind to go chasing ghosts using “blue car syndrome.” This is a yellow car anyway. I decided to use an organized series of tests to strategically lead myself down a logical path without wasting my time or the customer's money. By using my diagnostic experience and some of our high-dollar, high-tech test equipment - rather than model familiarity - I could document my findings and show the customer how we spent his money tracing down the problem. The first test I chose on this HT4100 throttle body injection (TBI) closed loop feedback control system was a good old basic fuel pressure test followed by an exhaust system backpressure test. Both of these yielded normal results. A quick verification of the secondary ignition system and a timing advance curve test followed. The secondary spikes were high enough to indicate a need for service but not high enough to cause the problem I was experiencing. The computer timing was normal. Next, I wanted to monitor the manifold air pressure (MAP) signal with one lab scope channel while using the other, with a current probe on the other, to see what the engine control module (ECM) was doing with the injector pulse. In this way, I could determine if the problem is logistical in nature; i.e., the ECM doesn't see the load or possibly a mechanical problem such as valve timing, restriction in the exhaust, valves sticking, etc.
The left side of the screen shows the enrichment pulses and the MAP signal while the right shows a screen capture of the injector, after the enrichment pulses, going from narrow pulse to a moderate pulse while the MAP signal shows relatively little change. With such a small change in the MAP signal, I'm immediately inclined to jump back to blue car syndrome and check that gummy line at the back of the base of the TBI unit, but then ... that wouldn't be fair. I decided the next step in continuing a logical diagnostic path would be to formulate a hypothesis about the facts gathered so far and devise a means to test the hypothesis. Basic backup strategy for this type of fuel system is to use the throttle body sensor (TPS) to substitute MAP in the event that the MAP signal gets far enough outside the ECM's calculated operational parameters for it to recognize the reading as erroneous. A trouble code check revealed no stored codes. Since I suspected the MAP signal as a problem (not necessarily the sensor itself), I decided to compare it to its closest cousin, the TPS.
The test revealed the MAP signal was significantly lagging the TPS. We're talking almost 1.5 seconds. Now, my last '82 Cadillac had a TBI with a slow MAP signal response due to a collapsed, gummy hose at the base of the throttle body. So to further eliminate the possibility of assuming this car had the same problem, I installed a vacuum gage right at the MAP sensor and compared the mechanical gage reading to the electrical signal of the sensor. The readings followed each other almost exactly, indicating the problem was actually a slow vacuum signal rather than a slow sensor. I traced the vacuum line to the throttle body where I found ... guess what? ... a gummy, collapsed vacuum line at the base of the rear of the throttle body. After replacing the line, I went for another road test with the scope still hooked to the TPS and MAP signals.
After explaining to the customer the need for the major tuneup and the O2 sensor, which were behind in their regular maintenance schedules, and then the diagnostic time tracing and repairing the vacuum signals - which he authorized without hesitation - I then explained that even though the car felt good and ran pretty well, I saw something on the test that had me concerned. I believed a throttle body overhaul was in order. He then uttered those cherished words, “Hey, you're the doctor, I trust your judgment.”
The passage was definitely restricted. After a good thorough soaking in the carb can, I blew the thing out and reassembled it. I then made all the necessary adjustments (Min. air, TPS, idle speed and ISC) and went for another test drive with the scope still attached. Figure 4 shows the resulting waveforms indicating the MAP signal is now following the TPS, as it should. The point is, had I followed the old school of thought - using seat-of-the-pants diagnosis and previous model familiarity - I would have likely missed a needed throttle body cleaning. The car runs a little better. (I mean, it's not bad for a 4100.) Now, everybody's happy.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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Figure 1 shows the injector pulse with the scope recording min/max and the MAP signal.
Figure 2 shows a comparison of the two signals during a hard acceleration test on an incline.
Figure 3 shows the test results, which were much more in line with what you would expect. The car responded much better and I would have called it fixed except the signals still didn't seem to mimic each other as closely as they should as far as response time. I took another look at the vacuum gage versus the MAP signal and saw the gage was still reading the same as the sensor and both were just a little behind the TPS.
I know how Mark Twain must have felt when he said, “I could live for a week on a good compliment.” I pulled the throttle body and tested the passage for the MAP sensor with a blowgun.
