A backfire sound is generally caused by timing but with sensors all around controlled by the computer, it could be anything. You didn't state the mileage but the timing chain could be worn throwing off the timing. At least you have a good offer from the trans shop.
Engine starts after 2nd attempt
2004 Chevy Silverado Z71 4wd 5.3L. Truck has never had a starting problem until I got it back from the transmission shop who replaced rear main seal, the plate the seal is installed in and the camshaft sensor. Now this transmission shop is one of the best in the business and honest to a fault, so I have no problem with them. Here's the problem. Turn the key and the engine turns but does not start. Try it again and it starts as usual. And it does this probably 95% of the time now. Sometimes if you keep the engine spinning long enough (5 secs) it will backfire or what sounds like backfiring. When I confronted them with this problem they thought it was the camshaft sensor. They explained that on initial start the camshaft sensor is in play and if a no start happens and crankshaft sensor takes over, or something to that effect. They wound up replacing it to no avail and put a known good one from another truck in,same result.They also replaced the pigtail the sensor plugs into. It's odd that this problem just happened after work was performed but I guess it could happen. They gave me a automotive shop's number and they'll pick up the charges if it's anything they did but I'd like to fix this myself if anyone has an idea what could be causing this.
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Check your fuel pressure with Key on engine off and key on engine on.
I would think of two things. a bad camshaft sensor. or maybe a bad connection to the neutral safety switch.
Thanks guys, this gives me a starting point. I’ll start checking these out in the morning.
Fuel pressure is good, sensor is good. Just won’t fire on first try. Second try, fires fight up!
K2500 might have a point but I would think that a bad neutral safety switch it would not crank at all.
Between the starts, is the key turned to off or just to the run position and back to start?
This is with both a hot or cold engine?
On the next attempt before turning the key, wiggle the shift selector or move the key a few times from lock to the run position.
Between the starts, is the key turned to off or just to the run position and back to start?
This is with both a hot or cold engine?
On the next attempt before turning the key, wiggle the shift selector or move the key a few times from lock to the run position.
WF, I’ll try that tomorrow, thanks!
so the repair shop replaced the rear main seal .. that tells me the crank shaft sensor under the starter motor was messed with or the wiring/connector . common problem like you have no start or erratic starts and many had no codes. the sensor has a encoder wheel in the location when the seal was worked on.WF, I’ll try that tomorrow, thanks!
why did they mess with the cam sensor ? if they messed with it then it needs a GM program re-due ..
Learn something every day. Plenty of YouTube videos on it.
Did you ever get your issue resolved? The shop I work at received a 2003 Chevrolet Silverado 1500 with a 4.8L & automatic transmission. The engine was replaced and the transmission was removed during the process and now has the same symptoms as your truck. The engine will crank but will not start on the first ignition cycle but is intermittent. If the engine is cranking longer than a few seconds a backfire will occur. Regardless of how long the first ignition cycle lasts the engine always starts on the second attempt. The range selector (AKA: neutral safety switch), cam sensor and crank sensor have been replaced by another shop but the condition did not change so they brought it to us. I have already performed a CKP variation relearn after hearing that the sensors were replaced but the condition did not change. There were two trouble code in the BCM [Body control module](B2961, B2615) but no other trouble codes in any other module. The ignition switch was inspected and we found that one of the switches that detects the key turned to the 'ACC/Run' position would not always show that the key was turned in the switch. The switch was replaced but the condition did not change although now the key position does read correctly after the switch replacement. I did notice that the 'PRNDL' indicator in the cluster does not illuminate in cluster but am pretty sure it is unrelated-I viewed live data from cluster and the cluster does show the correct position when the transmission is shifted through the different positions. I viewed live data when the condition occurred and confirmed the crank sensor is reading when the engine is cranking. My next test step is to put a fuel pressure gauge, ignition coil spark tester and scope to view the cam and crank sensors to see if this is a fuel or ignition related 'no start' condition when the issue is happening. If the test indicates an ignition related problem I was planning on looking over the harness and ensure all the grounds are in the correct locations and there are no pinched/chaffing wires that might have happened during the previous repairs but I would expect if the harness was damaged this issue would occur every time. The symptom does not seem to be related to temperature or the length of time the vehicle sits. Any help would be appreciated, this is a pretty unusual and frustrating problem LOL.
the engine was removed ? well you have to inspect all the wiring ,,, inspect all the PCM connectors ..also ... another issue the CPS next to the starter motor ...
Correct, the engine and transmission were both removed. I spoke with the customer over the weekend and they said this issue was happening before the engine replacement, so now I'm even more confused as to what is going on here lol. I did check the PCM connections for corrosion-everything looks normal and it looks like someone put di-electric grease on the connectors. I checked the main areas where the harness rubs like next to the alternator junction point and on top of the intake manifold but didn't find anything out of the ordinary. The crank no start did occur this morning with a backfire, the fuel pressure raised to 56psi within a second of cranking and the ignition coil is sparking so I've connected some test leads to the cam and crank sensor signal wires and to one of the injectors to see if the PCM is disabling the engine start for some reason but now it's starting normally so I likely won't be able to duplicate it until late today or tomorrow morning.
Sounds like a fuel delivery issue to me. Your fuel pressure came up after starting? It should lower, in my mind, when the engine starts taking fuel from the rail. Unless you mean it primed while it was turning over...
Turn the key on, but don't start the truck. Let the fuel pump prime the system. What is your pressure reading? Does it hold? I believe this generation truck still needs the ~60psi reading prior to starting to be in range and it should hold for a decent amount of time.
If the horn works, you can swap the horn and fuel pump relay just to eliminate one other thing.
Turn the key on, but don't start the truck. Let the fuel pump prime the system. What is your pressure reading? Does it hold? I believe this generation truck still needs the ~60psi reading prior to starting to be in range and it should hold for a decent amount of time.
If the horn works, you can swap the horn and fuel pump relay just to eliminate one other thing.
Sorry if my wording was confusing. With the truck completely off, before cranking the first time, residual fuel pressure was 38psi so I know the fuel pump is holding pressure in the system and the check valve is working correctly. As soon as the vehicle starts cranking the pressure raises to a peak of 56psi, I am using a digital fuel pressure tester so I'll check and see if the pressure is moving up/down slightly when the issue happens again-at the moment the truck is starting normally. I agree, I would expect the pressure to be moving up/down if the injectors are opening/closing while cranking. Key on Engine off pressure is 56psi. When the engine starts on the second ignition cycle the pressure is 48-52 at idle (Spec: 48-54psi Source: AllData).
the fuel pressure turn the key on no start 60-62 PSI...then the engine starts up fuel pressure is 55-58PSI...
many have a lower pressure around 50PSI eng runs but when the pressure is 47psi and lower not running...
many have a lower pressure around 50PSI eng runs but when the pressure is 47psi and lower not running...
Okay, I just manually actuated the fuel pump with the vehicle off until the pressure was at 56-58psi and the condition is still a crank no start. There was not a command from the PCM to turn the fuel injectors on while cranking and a cam/crank signal were both present and the signals are clear and do not appear noisy/erratic. The VTD (vehicle theft deterrent) system is part of the BCM and disables engine starting via the PCM by turning off the injectors. I'm going to check the security system and see if it can be bypassed in any way. The key position sensor was replaced but the passlock sensor was not so I'm going to reflash the BCM to re-learn the voltage code in case it was somehow lost/changed and will try to bypass the passlock if that does not work. From what I understand from the description of how it operates there is a resistor in the passlock sensor in the ignition switch that changes the voltage signal to the BCM and the BCM will allow/not allow an engine start depending if the voltage is correct.
Looking under the dash I found some aftermarket accessories wired under the driver side so I'm going to also uninstall those to inspect the wiring and put them on their own separate circuit. The VTD system says it operates on one of the data lines so it's possible an accessory may be wired to a CAN bus line and causing a faulty message to the BCM during the first ignition cycle making it unable to receive a correct signal. The accessories are an aftermarket radio and a "Bully dog" gauge tuner connected to the OBD port and jumped at one of the fuses at the left dash fusebox. I am also seeing the protective cover from the passlock sensor at the ignition switch missing and what look like pry marks around the sensor so I've ordered an entire ignition switch assy that should be here in the morning. I think I am close to finding out the main source of the problem.
Looking under the dash I found some aftermarket accessories wired under the driver side so I'm going to also uninstall those to inspect the wiring and put them on their own separate circuit. The VTD system says it operates on one of the data lines so it's possible an accessory may be wired to a CAN bus line and causing a faulty message to the BCM during the first ignition cycle making it unable to receive a correct signal. The accessories are an aftermarket radio and a "Bully dog" gauge tuner connected to the OBD port and jumped at one of the fuses at the left dash fusebox. I am also seeing the protective cover from the passlock sensor at the ignition switch missing and what look like pry marks around the sensor so I've ordered an entire ignition switch assy that should be here in the morning. I think I am close to finding out the main source of the problem.
1. Issues Starting the Vehicle
The most common symptom associated with a bad or failing crankshaft position sensor is difficulty starting the vehicle. The crankshaft position sensor monitors the position and speed of the crankshaft and other parameters that play an important role when starting the engine. If the crankshaft position sensor is having a problem, the vehicle may have intermittent starting issues or not start at all.
take the starter motor off and look up the CPS ....
The most common symptom associated with a bad or failing crankshaft position sensor is difficulty starting the vehicle. The crankshaft position sensor monitors the position and speed of the crankshaft and other parameters that play an important role when starting the engine. If the crankshaft position sensor is having a problem, the vehicle may have intermittent starting issues or not start at all.
take the starter motor off and look up the CPS ....
I scoped the cam and crank signals during the crank no start event and both signals are there and don't have any drop outs, missing teeth signals or a "noisy" signal. The cam/crank relearn was one of the first procedures performed when the vehicle arrived and did not make a difference to the condition so it doesn't seem to be related to either of these but I'll recheck the signal at the PCM side of the harness just in case there is high resistance or possibly a rubbing harness. I'll have to check with the customer if the cam sensor was replaced because it looks new.
Just a quick note that I have been following this discussion. I am the original owner of a 2001 Suburban LS (5.3L 4WD 190K miles) with the exact same condition as described in the first post on this thread. Have had it for almost 2 years now. 50% of time, fails on initial crank, but if you stop and wait 3 seconds, starts right up on the 2nd attempt 95% of the time - religiously. I have no auxiliary wiring and engine internals are 100% factory, no accidents. My vote is the cam sensor based on internet research but that's just a guess. Hope you solve this, good luck!I scoped the cam and crank signals during the crank no start event and both signals are there and don't have any drop outs, missing teeth signals or a "noisy" signal. The cam/crank relearn was one of the first procedures performed when the vehicle arrived and did not make a difference to the condition so it doesn't seem to be related to either of these but I'll recheck the signal at the PCM side of the harness just in case there is high resistance or possibly a rubbing harness. I'll have to check with the customer if the cam sensor was replaced because it looks new.
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