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C4 Dash earths. The dash cluster suffers from bad earth's and connections. The main earth on the left side of the block by the oil filter, is a known trouble area. This is the source of a lot of earth's for the vehicle. There is a dash earth on the earlier cars on the left hand door shut pillar, accessible from the foot well. I have not experienced problems with this one but it can effect the dash functions. Engine earths. Other earths run from the battery to the frame, some models have additional engine to frame straps. Ensure these are good and clean. Chevrolet sneakily designed an earth on the top of the bellhousing, very hard to see and check, and often it is not put back when a transmission is removed for any reason.
ECM. Most people think that this never goes wrong, but consider the harsh environment it lives in - sometimes hot, sometimes cold, vibration, etc. The unit can fail and will cause a multitude of odd symptoms. It is not that expensive or difficult to change and can, in a stroke, cure all sorts of odd problems. The ECM's up to 89 were inside the car, thereafter mounted in the engine bay above the battery. Oxygen sensor. The Oxygen sensor is, probably the most important sensor. As the computer uses this as its reference to see how the engine is running and alters injection and timing accordingly, it is essential that the sensor is good. It should, when scanned by diacom or similar, show wide variances of voltages, if it is 'lazy' then it needs replacing, it should be a healthy brown colour on the sensor portion in the exhaust flow.
MAF sensor relays. One major cause of problems without setting trouble codes is failing MAF sensor relays, these are different for the MAF power and MAF burnoff up to 86 thereafter the relays are the same for both circuits. These relays may show that they work but may not send the correct voltage to the MAF sensor. MAF sensor. These sometimes go bad, but ALWAYS check the wiring and the relays first, these are expensive items and a replacement is not necessarily going to fix the problem. If modifying these units to improve air flow, extra car must be taken not to damage the 'hot' wire in the centre of the unit. This is what is used to pass the air flow information to the ECM. As a further check the car will run with the 3 wire connector disconnected, but will set a trouble code. The computer goes into 'limp home' mode.
Throttle body and IAC. This is very often ignored in terms of maintenance. The rear of the throttle blades get very 'gummed' up with black deposits, these can also clog the essential air holes and vents as well as not allow the car to idle smoothly. A good clean, off of the car, with carb cleaner is advisable every year. Also remember to remove the IAC valve and clean the pintle and its seating area, this will help ensure a smooth idle. Clutch pedal switch. (manual) To start the car the clutch has to be depressed, this is an American thing, us Brits who are used to manual boxes know to check neutral before staring the car. The failure can be intermittent and you may think that the car is hot and needs to cool, normal symptom is a clicking relay but no crank, and some of the driver centre displays click on and off when cranking is attempted. This is also a symptom of a VATS failure. If the switch hasn't been bypassed already, you may want to do this, by removing the drivers hush panel and finding the connector (large purple and large dark green & white wire). Split the connector and jumper the connector to bypass the switch, use thick gauge wire as a lot of current is drawn through this and it will heat up if incorrect wire is used.
Fuel filters. These are normally always forgotten about and need to be changed at least every 2 years. A blocked filter can cause odd no start or stall conditions, which just go away if left for 10 minutes. Dirt can get into the tank and therefore to the filter and clog it. Good fuel pressure is essential for good running and performance. Filler drain boot. Take note of the above, the drain pipe in the filler area can get easily blocked and the filler area fill up with water and subsequently leak into the tank. Make sure its clear and drains effectively.
Opti Spark. On MY92 and early 93 LT1's this unit can cause problems due to bad design. The unit holds condensation and causes a shudder or misfire between 2000-3000 revs under light loads. There is a simple, inexpensive cure for this, unless the damage has gone too far, in which case a new upgraded unit is available - be warned it is very expensive. Bulkhead connector. This connector is the main harness to and from the ECM and most other components. The wiper well drain can drench this connector and if not weatherproofed sufficiently it can corrode the internal connections, normally the bottom connections are effected, which can result in inaccurate tach readings. It is always good to ensure that all connections are good and clean in this block.
DO NOT WASH LT5 & LT1 ENGINES. Do not wash either of these engines, the water will sit in the valleys and corrode all sorts of electrical connections which have been placed there for space saving. LT5 starters are VERY susceptible to this.
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This is intended a guide only there may be inaccuracies, please refer to the appropriate model year shop manual for correct procedures.