Cliff notes: my car intermittently behaves like it’s in reduced engine power mode but NO CODES.

The story:
I’m having a problem with my car however it’s not reporting any trouble codes so I’m struggling to find the root cause. In June my car started experiencing severe bucking and surging no matter what gear or rpm and eventually trouble code P0332 for the knock sensor came up. For the fix I decided to replace both sensors. I put everything back together and the car started-up fine with no apparent gas or vacuum leaks. After driving a couple hundred miles the car adjusted and the previous bucking and surging was all but gone with one exception. In the 1300-1800 rpm range I was getting some part throttle jerking which I attributed to my tune.

I was going to get the car fine tuned but got side tracked by a couple of other problems. First my rear Pfadt shock broke (another long story), AC went out (belt came off), my Accusump canister exploded (fortunately while the car was at rest in a parking lot).

Anyway, two months after replacing the knock sensors I started experiencing hard “cold” starts on the first crank attempt and reduced engine power like all cylinders not firing or running too rich - NO CODES. Regarding the cold start problem, the car would always immediately fire –up on the second attempt. Regarding the reduced engine power, the problem was only noticed at first intermittently then after every cold or warm start and eventually after any restart but NO CODES.

I decided to take my car to my good friends at QMS to look it over. On the way down the SWPS failed – the car may be cursed. Anyway, I got the steering wheel position sensor replaced and had my tune looked at while at QMS. Tuner said everything looked fine with no signs of vacuum leaks or MAF issue and that my LTFTs were good. He made an adjustment to take care of the part throttle jerking in the 1300-1800 range and I headed back home.

On my trip back and about 50 miles from my house the reduced engine power got significantly worse to the point where I couldn’t maintain highway speed. This time however only one code was displayed; P0175. I limped the car home and found that the O2 bung plug in my exhaust midsection had come out. I capped it and cleared the codes but the reduced engine power problem remained. After hours of reading, road testing and frustration I thought I found the problem – a sticking PCV valve. I replaced it and gas mileage improved and the reduced power problem while not nearly as bad still didn’t go completely away. So next I replaced the throttle body (my car has an LS2/7TB) with a brand new one, thinking maybe the throttle position motor was sticking. After doing the TB re-learn and after a long road test I again I thought the problem was gone but it’s just more intermittent and still NO CODES.

I had the battery checked at Autozone – it’s 100%.
Ran a tank of Techron and 93 octane gas over a 600 mile trip – no change.
Checked all wires and vacuum lines for damage – nothing visibly found.

I’m pretty confident that the computer warning codes are not shut off in my tune because when I pulled the plug to the MAF and TB while the car was running I got the expected codes and CELs.

I do notice that whenever the reduced engine power problem occurs that my instant fuel mileage reported on the DIC goes down until the problem goes away. Also, the rear of the car gets spotty more than usual. Again NO CODES – nothing.

I don’t want to keep throwing parts at this; please tell me what am I missing?

 

This may sound really strange but this is a factor in many C5 problems.

Does this happen when the car is wet or when the humidity is high?

Here is something to check. Spray water on the hood of the car. Lots of it like a heavy rain storm. Try to get it on the hood seals. Open the hood and see if any water has been leaking past the seals and dripping on the computer. If you find water on or near the computer it is possible that corrosion has set in somewhere on an electrical connection.

 

This problem just started occuring this summer and here like in most of the country it has been hot. The car is always garaged and I have only been caught in a down pour once during the period that this problem has been occuring. Of course in washing the car there is certainly the possiblity that water from that could cause some corrosion around the TAC module so I'll check to confirm. However, I would assume that I would get a trouble code if that was the source.

Thanks, all ideas are welcomed.

 

Several things can cause your car to go into "reduced power or LIMP mode"
We really need to know ALL your mods--would help greatly
Most common things however are 02 related---- If you have an 02 issue your ECM goes in "open loop fault" which is a kind of limp mode--No codes appear---- You can check your 02 funtioning with most any scanner--Make sure your front 02's are rapidly bouncing around .100 to
.950 millivolts---make sure the fuel sytem data reads "closed loop" after the car is fully warmed---and not open loop fault--When 1st started it should be in just plain open loop for a minute or 2 til it warms up then immediately change to closed lop

If your car is extremely modded you may have to make an adjustment to your "predicted airflow table"
If you have a cam--some cams require an adjustment to the "MAP PE THRESHOLD table" If it is out of wack you'll get intermitant rich codes

 

TTT for more opinions.

Additional info:

Removed the battery to check grounds and reconnected couple hours later - issue has not returned "yet". However, when the engine is cold (coolant less than 100 degree) it still does not fire-up on first attempt but immediately fires-up on the second.

 

Install a gauge at the fuel rail, key to on(not started), and check pressure / bleed down.

 

I was just going to post this...what is your fuel pressure? Once again, I think Lucky is dead on. Your highly modded going by your signature. You already said you ruled out vacuum leaks...so right off my head I think of these...

Fuel, Spark, and tune.

You could always have someone go with you, plug the laptop in and either monitor it or do a data log and then send that to your favorite tuner to have them look at. Shoot I bet if you logged it or had someone do it for you that a few tuners on here would have no problem taking a look at it for you. It more then likely would point you in the proper direction.

 

Agree and will check as soon as I get a fuel pressure guage. I had the problem come up again and it lasted about 3 seconds before running normal.
As for the tuner, Monday I'll be in Pittsburgh to have Ed Hutchings take a look. Hopefully I'll have this narrowed down by then but if not I'm confident Ed will sort it out.

 

Here's what I have after checking the fuel pressure at the rail.


Engine coolant temp 120, key on - pressure spiked to 58psi then dropped to 52psi and slowly dropped and held at 48psi.

Cycled key off and on (3 times) each time the pressure would spike to 59psi then drop and hold at 53psi.

Started the engine, pressure spiked to 60psi dropped to 59psi then back to 60psi and held while I let it idle for about three minutes.

Doesn't sound right to me, shouldn't the pressure be steady at 60psi?

Also, each time I turned the key on I could hear what sounds like a whine. The sound originates from behind the driver's seat area. I assume that would be the fuel pump???

 

Yes, the whine is the fuel pump indeed. Do you have any broken grounds? I had a ground that was barely hanging on by a thread and it would cause so many problems for me. Finally after 2 days searching I found it and it fixed all my problems. Trying to help sorry if I didn't =\

 

Okay, I think I'm getting somewhere with this. What should the pressure reading at the fuel rail be after the car has sat for awhile i.e. > 2 hours?? Mine reads 0.

 

I have a FP gauge on the A-pillar. It reads 58-60 consistently even at WFO. I run a Racetronics setup (Wallbro 255) with the Racetronics lead to the alternator setup.

The pressure fluctuation and the whine behind the seat are dead giveaways that you've got a pump on it's way out.

Get a new pump.

Good luck,

Kevin

 

Hey Kevin!

Yep, all signs certainly point that way. I just checked my injectors again for signs of leaking and didn't find anything unusual so I think you are absolutely correct.

Of course as my luck goes, I have a late build 2003 with the FFS which means it's going to be a pain in the (g)as to fix.

 

Okay, the problem has been fixed.

I bought a brand spanking new fuel pump and throttle body but neither was the problem.

It turns out that my issues were the cause of several little things. First some background, last year in April Isent my car off to add new heads and valvetrain which of course required a re-tune. It was tuned to run on the rich side specifically at throttle tip-in and WOT. Directly from the shop I had it sent to another shop for a complete re-paint so it would be all shiny and new looking so it was out of my hands for nearly 6 months getting re-painted i.e hacked by Autobuffs - another story. Not being willing to be seen in the car anywhere outside of my garage and the long winter, pretty much kept it off the road until May.

When I started to drive it this summer, over a year later, I noticed some bucking/surging/jerking in the off idle to 1600 rpm range which I attributed to the cam and the way it was tuned. I scheduled a re-tune but several other things mentioned above got in the way. Shock failure, knock sensor replacement.

When I replaced those sensors, which requires removal of the intake, I didn't replace the intake o-ring gaskets. However, since I didn't have any apparent signs of leaks I assumed I did everything right. Also, because the bucking/surging/jerking were now not as bad (bad knock sensors really cause bad drivablity issues too); I also assumed that remaining issues were still tune related. I finally had a re-tune but, as I mentioned in my first post in this thread, while the re-tune slightly improved things a little the problem had grown into the reduced engine power problem described.

Well it turns out that the reused o-rings on cylinder 1 and cylinder 4 were not sealing thus causing a leak and lean condition which my car responded to by by adding even more fuel. So I replaced the o-rings and thought great! However the problem was still there, to a lesser degree, but still there. WTF!!!

Aha! What I failed to also take into account was that by this time three things were causing the car to run richer (tune -still on the rich side, PCV valve, and an intake leak) which caused my plugs to be extremely fouled. Fouled plugs will make your car run like garbage. I replaced the plugs and BAM! All issues gone. No more reduced engine power. Drivability improved 200% and the final touches will be performed by Edcmat-l1 later this week.

Have a great day!

 

Hello I’m new to this forum.I have a 97 my car sat for almost 2 years I put fresh gas in it with new battery. It turns over but won’t start really no combustion. I check the fuel relay an all fuses I only hear my fuel pump kick on for not even a second. In use to a slight wine for a couple second no codes just reduced engine power. Please help