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Cranks But Won't StartPosted 11/19/2007By Brian Manley
We have had our share of these recently, and we always begin by verifying the customer's concern. If the vehicle starts, then we are faced with an intermittent concern, and our diagnostic path will lead us in a different direction. However, once you verify that the vehicle truly does not start, what is the first step you take? Let's look at three vehicles we've had recently, and we will walk through the possibilities together. 1993 Dodge Caravan - 3.3L - Automatic Transmission We verified that the engine cranked; it was strong. I popped the hood, and we performed a thorough visual of the under-hood area, looking for evidence of abuse, neglect, tampering or rabbit erosion. In addition, we did the following: Once we verified the above, we cranked the engine over again (with a charger on it at this point), but we really listened to how the engine sounded. If the engine had slipped a timing chain or belt, we would hear the engine fighting itself, or it would spin as though it has no resistance. Our engine sounded normal, as though it was missing spark or fuel.
Next, I grabbed a couple of spark testers and hooked them to two wires; we had no spark. I then hooked up a fuel pressure gauge to check pressure and take a sample; pressure was good and the sample looked and smelled normal. I went back to the scanner to look at the rpm while cranking, and it remained at "0."
Once I saw no rpm, I decided to go to the source of the signal. So I lifted the van on a rack and saw Figure 1 where the crankshaft position (CKP) sensor wires were. Those darn little bunnies (again)!
Now, this vehicle fell victim to an open CKP sensor harness, and repairing the wires resulted in a running vehicle; however, we didn't stop there. We finished a thorough inspection of the exposed wires and inspected all tuneup items before recommending repairs.
The next step on a vehicle that has spark, but no fuel pressure, is to check for fuel pressure and trigger to the injectors. Many technicians on the International Automotive Technicians Network (iATN) recommend not using propane to see if the engine is lacking fuel. Why? Because, for example, on a Vortec engine, the engine can start and run on the propane, but you've lost the window of opportunity to catch a weak fuel pump on a cold start that may be the root cause of the no start. Those systems are fussy about having full pressure!
1996 Nissan Sentra, Four Cylinder, Manual Transmission
This vehicle came to us with no fuel pressure, and a shot of propane made this engine come to life. This was a fun diagnosis because it really made us dig into the diagrams and do some sleuthing. Initial testing showed that we had B+ at the fuel pump, but no pump operation; but how could that be? Once we tested for B+ and ground at the two pump wires, we had 0 volts. Moving the ground wire to a good chassis ground showed B+ while cranking. Tracing the ground wire on the diagram revealed that the powertrain control module (PCM) actually grounds the pump when it wants it to spin, and there was no ground coming from the PCM. Figure 2 shows the terminal for the pump ground that is responsible for pump ground, and it has so much corrosion that it lost contact! We put a drop of solder on the terminal and tested the current draw of the fuel pump (see Figure 3) while testing pressure; all was in order, so we sold a rebuilt PCM.
1994 Mitsubishi Eclipse Four Cylinder Engine, Automatic Transmission
So, what else besides incorrect valve timing will cause the above vehicle to not start if it has spark, fuel, compression and injector trigger? Recent causes include a discrepancy in the camshaft and crankshaft signals (SYNC) and a severely plugged exhaust.
When I review many of the reasons for no starts that we've diagnosed, it becomes clear to me that there is not a single, flawless means by which to arrive at the root cause every time with no headaches. A good diagnostician must remember all of the repairs from the past, search iATN for fixes, and just plain step back and apply some logic through all of the possibilities when faced with a perplexing no start situation.
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Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
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Have a vehicle that cranks but won't start? Sounds easy, doesn't it? What could be more fun than grabbing the keys to a car that has that very concern? Gravy job, you say? Well, let's take a look.
If the scan tool does not communicate, what do you do? I had a vehicle come in recently, and we plugged in the famous aftermarket brand scanner with no communication, and then we plugged in the factory GM scanner and it communicated!
That being said, I use propane on a regular basis to see if an engine lacks fuel, and it is a safer test than soaking the intake with ether.
This vehicle had passed all of the tests through fuel pressure, spark, injector pulse and functioning MIL. We listened to it while cranking, and it sounded a little uneven at times, as though there was a timing issue. This vehicle had the following cranking compression readings in order from cylinders one to four: 160, 155, 155 and 170. Now, these are a bit high for our 5,800 feet of elevation so we pulled the upper timing cover off to check the cam alignment, and we discovered the belt was five teeth off! We turned the engine over by hand several times while watching the condition of the belt, and no teeth were missing. So we removed the engine mount and accessory belts to access the lower cover and timing belt adjustment tensioner. Setting the timing fixed the no start.
