OK, tell me how good or how bad my diagnostic skills are...

Yesterday my 99 vert started "coughing" or "hiccupping" at low RPMs under load; that is, between 500 and 1000 RPMs, trying to accelerate or climb a hill. Now, these "hiccups" weren't as gentle as a non-firing cylinder, they "kicked" pretty hard. It almost felt like I was hitting a small speed bump.

After determining this "kick" wasn't coming from the brakes or suspension, I assumed (maybe right, maybe wrong???) that early detonation was pushing one cylinder backwards; that is, trying to make it head back down-stroke before it reached top-dead-center. That would explain why the hiccups kicked back so hard, and I think would explain why it would also only happen during low RPMs under load.

Now, 2 questions... First, is detonation possible at low RPM's? I mean when you have it in 6th and you'r doing 25 MPH and you floor the gas. Could that cause detonation under certain circumstances?

Second, is it just a bad tank of gas? I filled up 2 days ago, and this started yesterday. Last night, I put a bottle of STP octane booster in and the problem diminished a LOT, although it didn't completely disappear.

Oh, I have a 3rd question... WHERE IN THE HECK IN SAN DIEGO COUNTY CAN YOU GET BETTER THAN 91 OCTANE GAS?????? :cuss

Thanks for any help!


:D

 

If it was detonation, I'd think that the knock sensors would retard the timing, at least enough so it didn't kick as hard as you described. You have a '99, have you ever changed the fuel filter? If the filter is clogged you can have all sorts of strange things happen as the fuel pressure fluctuates, but I thought that usually showed up at higher RPM's, not lower. Do you have any codes? If it's really kicking that hard you might find the reason in your DIC :eek:

 

Can't help with the gas question. I do not believe what you are describing is detonation. The knock sensor would detect any detonation and pull timing, you wouldn't feel a thing, except under WOT a loss of power. Also detonqation usually happens under load, in the heat etc. etc.

You could have a bad plug wire/plug or, as you mention, a bad tank of gas. Wait to see what the fuel cleaner does and also see if the car throws any codes. Sounds like a mis-fire of some sort to me.

Les

 

:iagree:

 

:iagree:

 

No flame intended here, but are you really trying to accelerate under a heavy load, such as up a hill, at less than 1000 RPM? I know these engines pull hard down low, but this sounds like lugging to me.

Assuming this is a new problem under the same conditions where you've had none before, it could be a throttle or crankshaft position sensor gone bad. I've had that before (not in the Vette) and it generated a very similar problem. However, it was an intermittent problem that was tough to reproduce. Are you throwing any codes when this happens?

 

Walford
I have been driving for 29 years (yea I know I'm old :) )and have yet had a BAD tank of fuel! In fact I ran regular (87 octane) in my 85 vette for 6 months and it ran just as good as it did with 97.
I run mid preminum (89 octane) in the 98 and have NEVER had a problem. Yes I run it very hard! :D I normally run 93 in the ZO6 but everynow and then it gets a tank of 89 and I dont see it having problems. So,, unless you got gas from someone selling it out of a 55 gallon drum on the side of the road, I'de almost bet a 6 pack that its not fuel related. My guess goes towards a plug or more than likely a plug wire!
I had two wires on my 98 that looked BRAND NEW and were BAD!!!! If you remove each one, examine the boots and wires for damage and check them out with an Ohm meter you should see around 300-600 Ohms. The two bad ones that I had were in the MEG OHM range! :eek: They were located on the passengers side and the EMI/RFI that they were emmiting caused a whole host of DIC codes to pop up. After I moved the bad wires to the other side of the engine (cure until I got my Magnacors) I have not had any more weird random codes like before!
Do you have the problem happen more on cold wet/damp periods? :rolleyes:

If you can answer yes to that question, you have secondary ignition problems! :thumbs:
Before you get to far into fuel filters and severe detonation problems, check out the wires. I would also check the DIC to see if you have any mis-fire codes set.
Yea,, I know. People really do get BAD gas and have clogged filters but I bet ya that there are not all that many. If you purchase gas from high volume stations, their fuel quality is usually pretty good and seldom present fuel quality problems.

My 2 cents.

BC

 

I just can't buy the bad gas theory, but you can get 100 Octane (racing fuel) from certain union 76 stations. Check their web site for locations in your area, I purchased some last year at $4.50 a gallon.