Always like when something gets fixed and gets reported back on how it was fixed!

 

Okay I'm back here, the blazer was still running rough after I made the adjustment to put the IAC counts up, also I want to mention I said before it always started the first crank for me before since the engine went in, well that's also because I fully prime the pump every time before starting it, when I don't, hot or cold, it will crank for a while and maybe stumble and start or maybe not until I re-prime it. I finally pieced together that these two things are related and got home today and checked my fuel pressure, sure enough, I found almost what I was expecting. The system will prime to around 62psi and holds well, I believe this is the system reaching regulated pressure. However when I start cranking it will immediately drop to about 50psi, and when it's idling it will be in the low 50's, see video below. This sure explains why it doesn't start while it's cranking, and the rough idle. It also would explain how the problem remained even with the engine rebuild, because the fuel pump isn't in the engine! Do you guys think this is enough to replace the fuel pump, or should I still do a deadhead test? Some background here the fuel filter was replaced last year, and a new spider with the engine rebuild was just done. To explain the video, I did not prime the system at all, I just went straight to cranking the engine as it has a good chance of starting first crank if I let it prime and I wanted to see exactly what it was doing while it was cranking and not starting.

 

Seems normal to me. Regulator in spider gets you about 58 psi after pulse before starting. As soon as engine has vacuum, fuel pressure regulator drops the fuel pressure. You can dead end the fuel pump if you want to verify what it really can put out.

Dont confuse statements you may find that the injectors require a certain pressure to "pop off" and deliver fuel This is only true for the old style fuel injectors.

That said, the dead end test on the fuel pump is a good one to do.

 

Thanks Les, it's indeed normal how it primes up to spec, however everywhere I look I'm seeing 50 psi is the magic starting number on these engines. Post #13 on this forum explains it well, the captain also says how "In the low 50's, about all you can expect, is a rough idle". Also I was looking at this site for further taking a look at my fuel pump, it again says it should be getting at least 50 psi while cranking, if it's under 50psi cranking it will not start. So my fuel pressure is obviously an issue here, it is the reason my truck won't start sometimes and idles rough, because it's not getting enough pressure while cranking or running.

 

You go ahead and do whatever you think is best - but this 50 psi problem you are speaking about is exactly what i was warning about. Later injectors don't require a pop off pressure. At least mine runs just fine at 50 psi. It was a problem with original injectors that was fixed by a complete design change. I have found the fuel pump pressure requirements published on this site by Captain to be extremely dated and not applicable to the updated spiders. Captain was my mentor but I say this based on my own experience. But you should have way more than 62 psi if you dead end the pump. So if you want to chase it go ahead. But I bet the fuel pressure regulator is controlling your pressure. If it was running rough because of a lean condition, the fuel trims would be off.

 

Thanks Les, I think I will end up doing the deadhead test and see what that yields... I'm just not sure what else could be causing this condition (rough idle and hard start with no prime). I was also thinking it could be the regulator controlling the pressure as the vacuum starts, but it just seems like it's choking itself out then, that would indicate the regulator is lowering the pressure so much that it's not supplying enough pressure so the engine will start efficiently, and that doesn't make sense. So, I'm really wondering now what fuel pressure any of you guys with a good running engine are getting while cranking and idling? Does your fuel pressure drop around 12psi from KOEO to cranking? From other posts online I'm seeing people have higher fuel pressure than me while cranking, it seems like it should be in the high 50's while cranking at least? And I don't think it should be dropping so much when I start cranking. About the poppet confusion Les, I have read about this and am under the impression IF you do have the poppets (which I do not), they will not open below 43 psi, however, what I understand, and I may be wrong is regardless of if you have the poppets or not, if the regulated pressure drops below 60psi, you will start to have problems, and again regardless if you have the poppets or not, the engine needs a bare minimum 50psi to start, this makes sense to me as in the video my fuel pressure was right around 50 while cranking and it was right on the cusp of starting. However, if I'm not understanding this please correct me!

 

I'll video my fuel pressure for you tomorrow under different conditions. Best thing I can see to do. Then you can decide if yours has a problem. I wont be doing the dead end test though - but it should be very high if I did (I would expect 80+).

I thought all was good after adjusting the throttle stop.

 

Thanks a bunch Les, that will be a big help here. You are right about the fuel trims as both short and long for both banks are 0, it almost doesn't make sense as I'm sure something is still up with it. I'm also getting down to the bottom of the barrel here, I mean the amount of air the engine is getting is within what it wants (my IAC counts are working), if it's not fuel related it almost must be spark related and I have a hard time believing that because all the spark plugs and wires are brand new, rotor and cap are pushing a year old (only driving 6 months out of that), and I only have 6 degrees of cam retard. In case it is something with the distributor I will be putting in that new aluminum one soon here and setting cam retard to 0, maybe I'll even throw a new ACDelco cap and rotor while I'm at it, but I really don't think that's the cause of what's going on here. I'm almost sure the hard start problem is a fuel delivery issue because it will only hard start when I don't prime the fuel pump first, hot or cold, and nothing else is changing in that time but the fuel pump pumping up pressure.

 

I have had a little time to reflect on what has been described, and it is bothering me that we caught both the cam sensor retard at zero (in your video) and the fuel trims (per post 38) all at zero. Also coincidence that IAC counts were zero? But here are the videos that I promised (and a couple extra).

First video was in Dash Command. Monitored Fuel System Status while idling after cold start on 85 degree Summer day. Took 3 min to go to closed loop. Note that for some reason I had to exit the data grid and go back in after cycling the key and starting the engine, in order to get Dash Command to read.

Went in Blazer to the parts store for the loaner fuel pressure gauge.

Second video was in Torque Pro after I got back home. Monitored both long and short term Fuel Trims for both banks while idling a warmed up engine. You can see the short term fuel trims going up and down. This is because of closed loop control of fuel mixture. Not absolutely sure, but I think the fuel trims would probably be zero when is in open loop. I'll find this out tomorrow morning after all cools back down. I am wondering that since your fuel trims are zero, that you may not be going into closed loop. But this is just a supposition. Let me do another video tomorrow morning when in open loop going into closed loop.

Third video was in Torque Pro. Monitored Fuel Pressure with coil disconnected during cranking. Note that may have gone down below 54 psi if oil pressure was not right there on a hot engine to keep the fuel pump running. Note 50 psi.

Fourth Video in Torque Pro. Monitored Fuel Pressure during a hot start. Note 50 psi.

Of additional note is that fuel pressure held at 54 psi for the full 10 minutes after shutting down. My cold starts in the morning are instantaneous.

Personally I think the first thing we need to do is verify if your vehicle is going into closed loop. You have the stuff to check it.

 

Wow thanks for this Les, this definitely shines some more light on what's going on here. So after I saw your video on the short-term fuel trim going up and down like that I knew something is probably not right with mine... I checked if it was going into closed loop on my way home from work and it indeed went into closed loop after a few minutes of driving, but not a peep from any short or long fuel trims ever, they all stay at %0.0. Hmmm, now I'm thinking a few things, first, this is definitely where the confusion was coming from in the last post, "You are right about the fuel trims as both short and long for both banks are 0, it almost doesn't make sense as I'm sure something is still up with it.", IF the fuel trims were trimming I feel like regardless if something else is wrong it would run better because it would be able to compensate either way to add gas or take away. I would also be able to get a better idea of what's causing the rough idle if I could see the fuel trims working!

Okay on your cranking video, it definitely seems like it likes to be at 54-55psi which is a much better cranking pressure than mine (49-50psi), yours also doesn't drop between when it primes to when you start cranking as mine does. I'm going to get the adapter tools to do a dead-end test and do the test in the next few days here, I definitely feel like the 49-50psi while I'm cranking is what's causing the hard start now. Given that it will prime up to 62psi and hold above 55psi for 10 minutes after I shut it off, could it really be anything other than the fuel pump? Something else here to note and that I'm a little confused about... so this is the second time I've caught this now, on my way home from work I had the tablet open with the data grid up, by the time I got home and looked at it (30 minute drive) I was sitting in my parking spot idling and the IAC counts were back pinned at 0, I turned the AC on and they went up to 40 or 50. Here's the thing, I turned it off then back on again and wallah I have 65 IAC counts at idle again without the AC on. WTH? What's changing in that time?