Gasoline Otto Engine No start/Misfire Diag Principles. - Land Rover Forums : Land Rover and Range Rover Forum
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post #1 of 4 (permalink) Old 04-18-2008, 07:01 AM Thread Starter
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Gasoline Otto Engine No start/Misfire Diag Principles.

All 4 cycle gasoline "Otto" engines need the same 5 things in order to run. It doesnít matter if itís injected, carbed, whatever. The following principals will help you diag a Crank/No Start and most misfire concerns on your Rover, Motorcycle, Lawnmower, etc. Most people have been taught Air, Fuel, and spark. Looking at these 3 will often point you down a wrong path. Always stick to the basics.

Letís begin.

1) You need air. You have to have air coming in, but also leaving the engine. Plugged air filters, plastic bags sucked in, clogged exhaust all can cause a no start or misfire.

Visual inspection of your air filter will be your friend here, also check the plumbing to the engine. Older Range Rover classics with the Fabric intake tubes had a problem with the fabric coming loose and suffocating the engine.

A vacuum gauge is an excellent tool to quickly access your air, among other engine concerns. They are cheap and available anywhere.

Using A Vacuum Gauge

This is a pretty darn good link about how to use one. Itís an excellent tool that is painfully underutilized in this day of Wiz Bang Trouble codesÖ.

2) Fuel. You need gasoline. Not only the right amount, but pressure to atomize the fuel. Atomizing is basically creating a mist. Picture trying to start a 2x4 on fire, and starting sawdust on fire. Same idea.

On Carbureted engines, engine vacuum is a big player. Most Fuel concerns on a carbed engine get diagnosed as an elimation of the other 4.

Fuel injectioned engines, this is where your service manual is going to come into play. Different manufactures have different specs and test procedures for different engines. No blanket statement will fit here. Specifics are the order of the day for test procedure and specs.

These gauges will work just fine. Required tool for diagnosis of misfire and no start. Smart money.

Confirm fuel injector pulse with a noid light. Make sure the computer is asking the injectors to turn on. | Shopping | Accessories | Product Detail - Great Neck 6 pc Noid Light Set

3) Spark.
You have to have spark. This is very easy to confirm. You are looking for a big, bright blue spark while cranking.

Lisle 50850 Ignition Spark Tester Order#*LIS50850 www.TheToolWarehouse.Net

This tool works just nifty. If the spark is big and blue, and pulsingÖItís safe to call the ignition system good.

Spark plug design will have an effect. Some people like Voodoo Magic spark plugs, and if it works for them great. For a diagnosis baseline, a smart mechanic will install OE spec spark plugs. Itís not unusual for the simple act of installing the correct spark plugs to cure misfire and no start concerns.

Confirm spark firing order with your service manual. Also, your vacuum gage can show incorrect firing orders.

4) Engine timing. The air, spark and fuel need to come together at the correct time. Timing can be confirmed with a timing light in most cases. However, most injected engines set their engine timing using Cam and Crank position inputted from sensors. These types need scan tools with real time sensor readings. These sensors are also used to set Fuel injection timing also. If you had a crank, no start that didnít have spark or fuel injection pulse on this type of systemÖ Smart money is on checking the Cam and crank sensor output.

Confirm engine timing at idle, but also confirm timing advance with raising engine rpm. This is another case of the service manual being your friend for specs and test procedure.

If you have timing and advance operation in spec. Timing is ok.

5) Compression.

Finally, you need compression. A rule of thumb is there should be a less then 10% difference from the highest to the lowest reading. The actual reading isnít as important as this %10 spread. Again, the service manual with have the procedure and specs.

Any of these will perform the task just fine

Every no start and drivability problem has been cause by one or more of these 5. Every one! Performing the tests to confirm what you have and what you donít will save you time and lead to a more accurate diagnosis as you can focus on what is not working correctly. (Fuel, Spark,Etc)
I would strongly encourage checking ALL 5 to get a good big picture view of whatís going on. Finding missing spark for example and stopping there in your testing can cause problems later in the diagnostic procedure.

I also cannot overstate the importance of a repair manual as a test and spec reference. Without knowing what specs you are looking for, and how to perform the testsÖWhy bother performing them, you donít know what you are looking for.

Donít let yourself get overwhelmed in the electronic gee wiz-ness. An engine is an engine, and these principals apply to all 4 cycle gasoline enginesÖ.Airplanes, lawnmowers, cars, motorcycles, etc.
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post #2 of 4 (permalink) Old 04-18-2008, 08:00 AM
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Good basic right up, Chris. Thanks. Bored this morning?

"In certain places, at certain hours, gazing at the sea is dangerous. It is what looking at a woman sometimes is." - Victor Hugo

Originally Posted by Elemental
Some guys play hard to get, I play hard to want.
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post #3 of 4 (permalink) Old 04-18-2008, 08:06 AM Thread Starter
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Yeah, a little.
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post #4 of 4 (permalink) Old 07-05-2008, 04:42 PM
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Thumbs up Thanks for great info!


This is the type of information I joined the group for... I am a self trained, learn by doing or by breaking and having to fix to avoid the long walk home mechanic... so basics on how the bleeding thing works (or should work in theory!) and then how to sensibly troubleshoot - including equipment/tools is priceless...
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