chevy g20

[kevnlou]

hi, im a new member, and i am in need of help. i have a 1990 chevy g20 5.7 v8 with tbi, and cannot get it to tick over and run. i have changed, plugs, leads distributor, distributor cap , and rotor, and had the injectors serviced and cleaned.i have spark, and petrol going into the throttle body and injectors, any help would be greatly appreciated . thanks. ive also changed the fuel filter

[Jsdirt]

First up, make sure every plug wire is where it's supposed to be. Trace every single one, and make sure the firing order is correct. FYI rotor spins clockwise ....

 

Set the engine to Top Dead Center COMPRESSION (NOT overlap), remove the cap, and make sure the rotor is roughly pointing toward the #1 cylinder. If not,(and the distributor was not removed for repairs) then it jumped time ... and you'll need to check for valve damage, worst case scenario.

 

If you have spark and fuel, it should at least "pop".

[kevnlou]

it will start, but dies straight away, it wont idle or rev atall. the whole distibutor was replaced and the engine was set to tdc, rotor pointing at #1 cylinder. i had the injectors ultrasonically cleaned and tested. but it does sound like the throttle body is sucking air when i try to start it.

[Jsdirt]

BIG vacuum leak? Was a line or hose left disconnected? Was the timing verified with a light?

 

Also make sure the IAC is working - I know Ford uses them, but I understand these units have them as well. It allows air to bypass the butterflies for idle, and controls the amount of air for idle speed.

[kevnlou]

im not sure about hoses, they all seem connected, im not a mechanic, or even a diy mechanic, so any help is fantastic, i just need it in lamans terms, lol

[Doug_Scott]

Look at all the vacuum lines that are attached to throttle body and attached directly to intake manifold. Some engines have a vacuum "tree" that screws into the manifold, generally behind throttle body. Pay close attention to the vacuum lines going to brake booster and pcv valve in valve cover. Look for any damage, softness, cracks etc. All lines must be connected to something. While there, pull dip stick and smell it. Does it smell like gas? What is oil level? If oil appears to be thin and runny, is over full, and smells like gas, you may have a gas in your oil. Either change it right away, or, remove pcv valve from rocker cover, leave vacuum line on valve, just pull it out of cover and try to start it again.

 

Van is old enough to be more like a carb engine than a fuel injected engine.

Did the van run before the distributor was replaced? GM HEI in those days were very fussy about distributor ground. If the ground connection (there is no wire, ground connection is made by physical connection of housing to engine) is poor, it will cause the ignition module to fail. You may want to try advancing timing a bit, this is done my turning distributor counterclockwise. Try not more than about half inch (use vacuum advance housing for reference). If the engine starts to crank very slowly or seems to stop for second, then cranks a bit more before stopping again, you have gone too far, move distributor clockwise half inch at a time and try again. If this allows engine to idle, then set timing with light and see if it works now.

 

If the engine starts, and immediately dies, check power to fuel pump during startup and running. Check the power at the fuel pump not the relay. Connect volt meter or test light to power wire at fuel pump(pump is inside the tank, connect to wire that goes into tank). Pump will have battery voltage when you turn key on initially. Start truck and watch voltmeter. When truck quits, you will need to know what happened first, did the truck quit, then voltage went to zero, or did it go to zero first then truck quit. If it goes to zero first, then you need to check the fuel pump relay, and whatever sensor it uses to determine if the engine is running. They used to use the oil pressure sending unit, but I do not know if that is what they used on your vehicle.

If the voltage went to zero after the truck quit, that is normal.

 

I do not recall if GM used the inline resistor wire with their HEI ignition systems, Engines used to use a system to reduce ignition power supply to approx. 9 volts back in the day. Dodge used a ballast resistor to do this, most others used a special type of wire in the harness to do this. This wire or ballast resistor was bypassed during startup (when engine cranking, battery voltage drops, so ignition system would never see 12 volts at start up), and once key was released, the bypass was disconnected and voltage for ignition went through ballast resistor or that wire in the harness. On a Dodge, you could just slowly turn the key back from start until the starter released, then hold the key there and it would stay running. I would give that a try just to see what happens. Don't touch the gas, just see if it will idle. Starters have a one way or over running clutch on the gear that engages the flywheel or ring gear to protect the starter from damage when you do this.

[kevnlou]

i think ive narrowed it down to the map sensor, as when i remove the pipe that goes into it, the engine will run, and when i replace it the engine cuts out. now my question is this, what is a map sensor for, and can they be repaired, cleaned, or do i just need a new one.

[Jsdirt]

I'm not 100% familiar with this system, but on the 80's Fords, the MAP sensor decides how much fuel the engine gets. A bad one will make it run like crap, or not at all - either pig rich, or very lean.

 

I'm sure someone will chime in with better info eventually ....

[Duelin]

The MAP sensor tells the ECM how much of a load is on the engine. It does this by measuring the vacuum and turning that info into an electrical signal so the ECM can understand it. The ECM uses that info to adjust the pulse width on the injectors. Also to adjust the spark timing accordingly. When you disconnect the MAP sensor vacuum source the ECM reverts to a default setting (On most vehicles). Now the question is. How is you vacuum readings? Are they what they should be. If not then the MAP sensor is going to send a bad signal to the ECM. And the ECM is going to make adjustment on bad info and you end up with an engine running terrible. The next question is. Did you time the engine correctly? Did you use a timing light and if you so did you disconnect the spark advance wire before timing it. If it is not timed correctly that will cause your vacuum readings to be off.

[Jsdirt]

^^^ Now that's the description I was hoping for. Jogged the ol' memory.

[kevnlou]

i put a new one on, but still the same, take pipe of to it, engine runs, put it back on engine dies. i have no engine lights on, so i will check all fuses and relays. wiring in van is shocking, has had alot of bodge jobs done.i cant do timing et as cant get the engine to tick over properly, also, there is no other wire on the distributor other than the 4 pin and 2 pin connectors.

[Doug_Scott]

Somewhere in the near vicinity of the distributor there should be a single green wire with a connector on the middle of it. You just need to unplug it.

 

Can you drive the truck with the MAP sensor disconnected? Can you leave that vacuum line on, but disconnect another line instead? If you can, try that and see if engine will run.