: Engine Hesitation/Miss-HELP!



05_CTSV
07-21-08, 02:10 PM
Guys, been feeling a hesitation/mis while driving for about 2 weeks now, first started out only at certain rpm (3000) and would only feel it once in a while. Has progressed to the point of any rpm, any gear, can really feel it while maintaining constant rpm, but starting to show on hard acceleration. It feels like it falls on its face when accelerating. Took her in to dealer last week and they drove it and claimed "runs smooth as silk" and maybe I had a bad tank of gas (even though I have been through 4+ tanks since problem started). I hasn't thrown any codes as of yet so they say "there is no way to begin to diagnose the problem". I disconnected the MAF sensor and drove it around to see if maybe dirty MAF was problem, no change (cleaned it anyway). To me feels like stumbling associated with not enough fuel (based on old carb engines). Where should I go from here (other than new dealer :-), fuel filter, injectors, ??? Thanks in advance for your thoughts!

Cadillac Tony
07-21-08, 02:27 PM
I'd start with plugs & wires. The wires are a PITA to get off without a plug boot puller (don't ask me how I know), so rent or buy one to save yourself skinned knuckles and cursing. Wires and plugs are cheap for this car, so if there's any indication of a problem, replace them and unplug the battery to reset your Long Term Fuel Trim data, then drive it for a bit and see if that fixes it.

If that's not it I'd try the fuel filter, then maybe measure the pressure at the rails to see if it's a sending unit issue. Start cheap, then move up if need be. A fuel filter's only 15 bucks, so if you're still on the original one it's a good time to change it anyway.

05_CTSV
07-21-08, 02:34 PM
Tony,

Thanks for the reply, one of the first things I did was unplug the battery and pulled out the PCM to make sure all the connections were tight as the dealer had replaced that about 2months ago for a bad ac connector. So hopefully the Long Term Trim was wiped out. I will check the plugs and wires, didn't immediately jump to mind since my car has only 34K on her, but I will check anyway. I just talked with a friend of mine who is a GM tech at a Chevy dealer and he offered to put some sort of Active Fuel Management Diagnostic tool on it that I guess checks all of the injectors 1 by 1 so I think I will take him up on it. If worse comes to worse, I'll take a drive up from Orlando and have your guys take a look at it. Thanks!

StealthV
07-21-08, 09:14 PM
What's the fuel rail pressure?

05_CTSV
07-22-08, 08:19 PM
Stealth- I put the pressure gage on and it read 55psi at idle, not sure if it is supposed to change but it did not as I slowly increased revs, or short 3/4 throttle revs, it just read 55psi constantly.

darkman
07-22-08, 08:49 PM
The shop manual indicates the acceptable range to be 55-62 psi. It does not discuss varying the rpm during the test.

StealthV
07-22-08, 10:23 PM
Stealth- I put the pressure gage on and it read 55psi at idle, not sure if it is supposed to change but it did not as I slowly increased revs, or short 3/4 throttle revs, it just read 55psi constantly.

Those readings look good as the pressure should be constant.

The recent dealer work raises a flag in my mind.

Out of the blue idea - Bad engine mounts?

05_CTSV
07-23-08, 08:04 AM
Thanks for the replies, I agree the dealer PCM change sticks in my mind as the beginning point of all of this mess (however I have another pcm that I swapped in and it made no difference at all so I swapped it back out). I am going to pick up a fuel filter and change that out this week as a starting point. If that doesn't help hopefully the Fuel Management tester can diagnose something. I understand the dealer's point that without a code being stored they have many different possibilities to start with but I would figure they could at least take the time to rule out a couple of simple ones on a $54,000 car, instead of playing the "bad tank of gas card". But what should I expect from the same folks who told me that my aftermarket air intake was the source of mysterious air conditioning hiss that happened to show up right after they supposedly evacuated the R134A and filled it back up as part of the PCM problem. (I had to physically help them diagnose the problem and it ended up that they hadn't filled the R134A to the proper level the first time and had to do it again).....rant off.

05_CTSV
07-23-08, 08:07 AM
Stealth- I had them check my motor mounts about 2months ago, and they said all was fine (If I can believe that after all this.....) but I really don't exhibit the long dieseling shutoff. When I have the car in the air to replace the fuel filter I will check out the mounts and see if they are leaking. Thanks

StealthV
07-23-08, 09:43 PM
If my memory is correct, the fuel filter is inside the tank and part of the fuel pump assembly. If so, it's not your typical in-line, off the frame rail, filter swap. :(

darkman
07-23-08, 10:25 PM
According to the shop manual, the fuel filter is in the engine compartment under the trash can lid. However, the shop manual also says that: (1) there is no replacement interval specified for the fuel filter, which should be changed only if it is found to be restricted; and (2) a restricted fuel filter means that the gas tank should be inspected and/or cleaned.

05_CTSV
07-24-08, 08:17 AM
According to the shop manual, the fuel filter is in the engine compartment under the trash can lid. However, the shop manual also says that: (1) there is no replacement interval specified for the fuel filter, which should be changed only if it is found to be restricted; and (2) a restricted fuel filter means that the gas tank should be inspected and/or cleaned.

Hmmm, maybe there are multiple filters on this thing, I am pretty sure Stealth is right that there is a "sock" filter as part of the fuel pump assembly in the tank, I haven't heard of the one on the fuel rail (i am guessing), but I know there is one just inside the frame rail under the passenger rear seat as I crawled under the car an looked. It is the typical inline filter with the 5/8" nut on one end and the dual snap in fittings on the other. Well I guess I may be able to change 2 out of 3. Another question, when it feels like it leans out/starved/ miss, would/should the PCM be storing a misfire count/other code whether it be real or pending? Would this be something a Tech II be able to find?

darkman
07-24-08, 09:31 AM
I must appologize, the filter is underneath the car just behind the engine compartment. I was confused by the shop manual's instruction to remove the trash can lid as part of the process.

The in-tank "filter" is referred to a screen, which presumably would only become restricted by larger partcles of solid contamination.

05_CTSV
07-24-08, 11:01 AM
I must appologize, the filter is underneath the car just behind the engine compartment. I was confused by the shop manual's instruction to remove the trash can lid as part of the process.

The in-tank "filter" is referred to a screen, which presumably would only become restricted by larger partcles of solid contamination.

No problem, I appreciate all of your help, I am at a loss right now and just trying to take care of the simple things I can, before fighting again with the dealer.

05_CTSV
07-25-08, 08:53 AM
OK-just put the new fuel filter in last night, no difference at all. Checked the motor mounts while I was under there, no leaks at all they look great. Checked all of the plug wires to make sure they were tight on the coils, all ok. Couldn't check end on spark plugs as engine still hot. Well, hopefully the Fuel Management System tester will be able to tell me something tomorrow........

05_CTSV
07-26-08, 02:47 PM
UPDATE- Just got back from my friend at the Chevy dealer. He put the active fuel management tool on and it said there was a gross leak during the fuel rail bleed down test. Basically you start the car, and it reads the pressure at the fuel rail (mine was at 55psi). Then you shut car off and turn key to "on" position and it runs a 45sec test to measure bleed down at the fuel rail. Mine went from 55psi to 48psi in 45sec. Apparently this is out of spec and it would not even allow us to run the injector tests. So we did the injector flush anyway just as another thing to cross of the list. Well needless to say still no different. My tech friend thinks maybe fuel is bleeding back thru the fuel pump. I am not sure how that would give me a feeling of engine miss when the pump is running. I guess I am at the mercy of my fine Cadillac dealer and will have to start to push customer service hard if they "can't find the problem". Also noticed that when they replaced my truck hinges and struts, that my trunk is no where near being flush when closed, add another one to my list.......................ARGH!

qqik4u
07-26-08, 08:17 PM
:eek:Don't feel bad - I have the engine hesitation or what I call bucking problem for more than 4 months now and have been going back to Lindsey with Bill - Poor guy - I am driving him crazy to fix this, but seems like it might be in th tune or the maggie (Noooooo :eek:) -

So if Bill can't find the problem at the world renoun dealership - I am at the mercy of my tune guy (tony's corvette tune in MD) - Will take it in two weeks - hope he can help. I have done all the above you have mentioned and nothing yet (even changed the spark wires to Callaway spark wires and performance plugs as well, but the list goes on).

Again - hang in there I am getting a bit frustrated and want this problem resolved well before the VII day.

CBVLS6
07-26-08, 09:36 PM
I hope you sort it out... I had this problem when my plugs got fouled after my wires melted/frayed from the headers. I would double check your wires also.

05_CTSV
07-28-08, 08:54 AM
:eek:Don't feel bad - I have the engine hesitation or what I call bucking problem for more than 4 months now and have been going back to Lindsey with Bill - Poor guy - I am driving him crazy to fix this, but seems like it might be in th tune or the maggie (Noooooo :eek:) -

So if Bill can't find the problem at the world renoun dealership - I am at the mercy of my tune guy (tony's corvette tune in MD) - Will take it in two weeks - hope he can help. I have done all the above you have mentioned and nothing yet (even changed the spark wires to Callaway spark wires and performance plugs as well, but the list goes on).

Again - hang in there I am getting a bit frustrated and want this problem resolved well before the VII day.

Thanks, I am just frustrated that no one at the Cadillac Dealer seems to want to TRY and figure the problem out. I just dropped it off again this morning, took a ride with the shop foreman again, told him what was done and his response was "well I can reset the fuel trim tables" since you have an after market intake on it, it probably goofing the MAF sensor up". Hmmm I can disconnect the battery myself...I told him I have had the K&N CAI on for over 2 yrs and the problem just started 3weeks ago. And that to rule out the MAF, I drove around for a while with the MAF sensor unplugged (so it would run on the tables only) and I made NO difference. He must not have liked my comments (even though I am trying to help him rule things out) because he didn't say another word to me. He told the service writer that they were going to perform a fuel rail leak check, I can bet my paycheck that mysteriously it will come back fine. I am to the point of buying my GMPP 4months early just so I can take it to the dealer of my choice. I love how some service techs treat people like they are stupid in the hopes they will just go away....present company excluded of course :-)

Cadillac Tony
07-28-08, 09:26 AM
I noticed you said, "Buy a GMPP 4 months early"- does that mean you're still under factory warranty? If so, get that baby up here to Leesburg!

I talked to one of my Techs who's pretty savvy with the small blocks (used to be a Chevy tech, so he's worked on lots of 'Vettes)- he suspects either the check valve on the in-tank pump letting fuel bleed back in or a stuck injector. Either way, we'd be glad to take a look at it for you and put you in a loaner while we check it out.

05_CTSV
07-28-08, 09:31 AM
I noticed you said, "Buy a GMPP 4 months early"- does that mean you're still under factory warranty? If so, get that baby up here to Leesburg!

I talked to one of my Techs who's pretty savvy with the small blocks (used to be a Chevy tech, so he's worked on lots of 'Vettes)- he suspects either the check valve on the in-tank pump letting fuel bleed back in or a stuck injector. Either way, we'd be glad to take a look at it for you and put you in a loaner while we check it out.

Tony,

Yes I am still under factory warranty until Nov. Hopefully my current dealer will not be jerks and try to void my warranty, and if they cannot fix my problem today I will bring it up to you. My tech friend the Corvette tech for the area, and he said the exact same thing, check valve probably first and stuck injector next. I will certainly take you up on your offer should the outcome here be anything less than satisfactory. I appreciate it very much!