This 1994 Honda Accord came in with the complaints that the engine would develop a bad tip in acceleration problem as the engine warmed up. Also after the engine was warmed up and the throttle was opened once the idle would become very erratic and the engine would stall if the throttle plate was not kept open and continually feathered. I tried reading codes while the condition was present and could not establish communication between the scan tool and the PCM.
I also tried reading codes by installing a jumper wire in the two wire diagnostic plug under the passenger side of the dash.
The diagnostic charts were a little unclear about how the light should react if there were no codes. In my case the light would come on but it would not flash. I thought at the time it was significant but after fixing the problem and making sure there were no codes with the scan tool the light still would not flash, on steady when the diagnostic connector was jumped.
In testing for a system that will not enter diagnostics I found a test that involved disconnecting three different engine sensors. The first on was the MAP sensor. Disconnecting it did not re establish communications.
Next up was the TPS. Same reaction as above.
The last sensor was the ERG position or lift sensor. Disconnecting this sensor did re establish communications between the scan tool and the PCM. It did not make any difference in how the engine ran though.
It seemed unlikely that an EGR position sensor would go bad under the warm up conditions and then heal itself after a long cool down. What if the EGR valve was going full open after warm up. Disconnecting the vacuum hose at the EGR valve produced a distinctive pop of the EGR valve closing. I started the engine and it ran normally. While the engine was running, I reconnected the vacuum hose to the EGR valve and the engine continued to run normally. Opening the throttle induced the tip in acceleration problem and subsequent stalling condition.
I traced the vacuum hose back to it’s source and all appeared to be okay. I disconnected the harness to the EGR valve control solenoids and the problem went away. Reconnecting it and raising the engine rpm’s would bring the problem right back.
I then took a really close look at the two hoses that come from the engine and the EGR valve and eventually connect to the EGR control solenoids. The hose connecting to the EGR valve is labeled #16. The hose connecting to the intake fitting was marked #24.
I followed the #24 hose from the intake port along the metal tube to where it reconnected to a rubber vacuum hose that was labeled #16. You have got to be kidding me the problem is that simple.
The hoses looked as if they were properly positioned and has a smooth flow from the engine to the connection points on the strut tower. However they were crossed. The #16 hose belongs on the upper pipe and the #24 hose connects to the lower pipe.
The above picture has to hoses connected incorrectly. The picture below shows the correct hose connections.
A couple more pictures of the hoses and how the connect and are routed.
The really sad thing about this car is that the engine was changed in an attempt to correct the problem. Along with the distributor and the PCM. It did have some very real problems with the coolant temperature sensor, spark plugs, spark plug wires and distributor rotor. All of that had to be corrected before the engine would run well enough to get to this diagnosis.
In case you wanted to know what was actually happening, by design the EGR valve is controlled by the PCM. The engine has to be up to operating temperature before the PCM will command the EGR valve to be opened. It is also varied according the engine rpm. With the hoses crossed the EGR valve was being commanded to open but the system could not properly vent the vacuum to release the EGR valve and allow it to close again. The more the throttle was opened the further the EGR valve was opened and a large internal vacuum leak was created. This caused the tip in acceleration problem and the resulting stalling condition.
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