QuestionI have a 1987 GMC Suburban with the 5.7L throttle body injected engine, with about 118,650 miles currently. I purchased the truck used in April, 2004 with about 115,400 miles on it. The engine had been replaced 3 years ago, at 112,300 miles. I have no record of how many miles were on the replacement engine, nor where it came from. The truck had sat off and on for the 3 years since the replacement engine until I purchased it, but it now logs 250-500 miles a week.
The truck ran very well when I first purchased it, but about a week later it began to idle rough (tach fluctuated from 550-750 when idling) and would occasionally shut off when put into gear right after starting (automatic transmission). I replaced the in-line fuel filter to rule out possible fuel blockage there, with no improvement.
Later, the engine service light came on while driving at highway speed, I stopped the truck, but could not identify any new problems beyond the rough idle. I shut the truck off, and restarted – the service engine light did not come back on. About a week later, the situation repeated itself, and I used a recently purchased code reader to check the trouble code. The reader indicated code “32” which my Chilton manual indicates as “The barometric pressure sensor circuit voltage is low.” Restarting the engine again cleared the service engine light, but the rough idle was still present. At this time, the truck began to run rough at speed as well, particularly when climbing hills at 55-60 mph in overdrive. The problem was most pronounced at around 1000-1400 rpms, and would be less noticeable when I shifted down into drive (2000-3000 rpms climbing at 55 mph).
This scenario occurred about 3 more times that week, and the truck shut-off and would not restart the 4th time. When the truck shut-off, the engine light came on and all the electrical was working. The engine would crank but would not fire. My code reader indicated codes 13 (“The ECM has detected a problem in the oxygen sensor circuit”), code 32 (“The barometric pressure sensor circuit voltage is low.”) and code 42 (“Electronic spark timing bypass circuit is grounded or low.”). I found that I was not getting any spark to the plugs, or the distributor, and found that the coil had shorted. Intending to keep the truck for a long time, I went through and replaced the Engine Control Module, the Electronic Spark Module (in the distributor), the rotor, cap, coil, plugs and plug wires. After this, the truck ran like brand new with idle rock steady at 750 rpm.
After about 3 weeks, the idle began to run rough again. Since then, the service engine light came on again, giving a code 32 reading. I did some web research on troubleshooting barometric pressure sensors, and ended up checking the MAP sensor according to the instructions I found in a November 1997 edition of Wells CounterPoint (http://www.wellsmfgcorp.com/counterpoints/Counterpoint1_1.pdf). The test indicated the MAP was good, but the rough idle problem is still there, and the engine light has come on in the same manner twice more. The problem during hill climbs has also returned, but is not as pronounced as before.
I am a DIY mechanic and plan to hold on to this truck for years (it is hard to find 4x4's with room for the wife, 5 kids, and dog that you don't have to mortgage your house for!), so I would like to understand what is going on with the engine during normal operation, and what might be going on now to give me the trouble I've indicated. To that end, I was wondering if I could get some help with the following specific questions:
1) Is the barometric pressure sensor on this engine integral with the MAP sensor or is there a separate sensor I have not yet located?
2) If the barometric pressure sensor is integral with the MAP, was the test I performed adequate to verify proper sensor function? If not, what other tests do I need to do to verify the sensor is working properly, or should I simply replace it now?
3) Is it possible that the pressure sensors (MAP and / or Barometric pressures sensor) is altitude specific? This seems unlikely, as the research I've done implies that one purpose of these sensors is to help the ECM adjust the engine for altitude. However, we are right at sea level, and as I stated before I have no history on the replacement engine.
4) Outside of the sensor, what other conditions could cause the sort of symptoms I have described?
5) What other troubleshooting tests should I perform to isolate and identify the root cause of this problem?
6) Outside of the problem I described above, are there any known engine issues I should be aware of with this truck or engine?
AnswerThe FIRST think we need to know is if the spark plugs are BLACK CARBON FOULED ?. Pull them and let me know. If they ARE this indicated to much fuel within the combustion chambers (cylinders) of which is usually caused by an intermittent faulty MAP sensor and or as what always happens with old vacuum lines where the vacuum line from the MAP sensor to the throttle body is restricted due to inside contamination and or the hose is much softer as compared to a new hose and this weak hose tends to collapse (Suck it self flat) thus causing a vacuum restriction of which sends a signal to the ECM (computer) from the MAP sensor telling the ECM that more fuel is needed, Thus causing excessive fuel to be delivered to the combustion chambers.
The barometric sensor if used on your vehicle is RARELY plays any significant role in the vehicles driveability.
Since you have several CHECK ENGINE codes and of the components that you have replaced, Warrants a thorough look to be ABSOLUTELY POSITIVE that all of the engine ground wires (usually black in color are properly attached to a quality ground point(s) on the engine block as if one is loose this will cause your type of problem.
Also be very careful as on some engines they use a ground point on the backside of the enine block ahead of the transmission usually on the backside of the cylinder heads and this ground wire(s) is very commonly overlooked as it is a pain in the BUTT to get to after the engine has been re-installed and or they just FORGOT to refasten it.
Let me know.
autohelp in NM.
