Predetination?

[blkstangman88]

my 88 GT is predetinating at low rpms and while in town. but when im on the free way or "racing" the car runs fine. what could cause this? i had the tfi module replace about a year ago, could it be going out again?

 

[drgn5.0]

lack of fuel at partial throttle?

 

[BlackGT89]

I don't think it would be the TFI module. Usually the module gets hot and the ignition just cuts off all the sudden. Then after it cools (usually on the side of the road) it will likely start and drive okay for a while, but the pattern keeps repeating and the intervals get shorter and shorter until one day you are stranded. There is a little info at this website about detonation and preignition (not predetonation):

http://www.hastingsmfg.com/Service Tips/detonation_and_preignition.htm

"Detonation can be caused by:
Lean fuel mixture
Fuel octane too low
Improper ignition timing
Lugging
Carbon deposits
Excessive milling of heads or block, which will increase compression ratio. "

"Pre-ignition, as the term suggests, is the ignition of the fuel-air mixture before the regular ignition spark from the spark plug. If the regular spark occurs shortly after the pre-ignition, the colliding of the two flame fronts will cause a pinging noise. Preignition causes loss of engine power and can cause severe damage to pistons, rings and valves."

"Pre-ignition can be caused by:

Carbon deposits that remain incandescent
Spark plugs too hot a heat range
Spark plugs not firmly seated against gasket
Detonation or the condition leading to it
Sharp edges in combustion chamber
Valves operating at higher than normal temperature because of excessive guide clearance or improper seal with valve seats.
Overheating
Ignition crossfiring. Induced voltage in spark plug wires that run parallel to each other for long distances "

 

[blkstangman88]

wow, thats a nice explanation.....i think ill check a few things and get back to ya guys on what i find...thanx

 

[hllon4whls]

GM sells a top engine cleaner that will clean all of the carbon off the top of the pistons and the bottoms of the valves. You pour it into intake vacuum port while the engine is running untill it chokes it down. Then you let it sit for about 1 hour. Smokes like hell when you crank it back up.

 

[blkstangman88]

whats that stuff called?

 

[L8 MUSL]

whats that stuff called?

Yeah, what?

 

[Michael Yount]

Have you pulled the codes? A malfunctioning egr system often causes part throttle detonation. The egr system actually cools the combustion chamber allowing a leaner mixture and more timing to be run. With a malfunction and a hotter chamber, that same leaner mixture and more timing combination often leads to part throttle detonation. Full throttle isn't a problem because at w.o.t. the car goes into open loop, and the timing and mixture are controlled by other parameters. But first, I'd pull the diagnostic codes out of the computer.

 

[blkstangman88]

no codes came up. i know this seems weird for a car with 233k miles, but no codes came up when the shop put a computer on it.

 

[DMAN302]

I have simular problem with my 88GT, I have found when I drop timing back to 10Deg the car seems to respond much better at lower RPM when throttle is applied. This could have somehting to do with mods and car being SD, or carbon build up, or overall condition of the internals..my engine also has higher milage and it definately likes higher octane and lower timing than my old 89.

 

[hllon4whls]

The stuff is called top engine cleaner. I dont have a part number, sorry.

 

[HISSIN50]

i think sea foam is the same type of product (try a search - there was a huge thread or 20 about it a year or so ago).

on DSM's, many folks had problems with the stuff (i dont remember which flavor - if i saw the name i would remember. it was not what Hllon mentioned), so be forewarned that there could be side effects. i have no personal experience with it, but it seems to get good feedback from stangers.

Blk 88, is your fuel system stock? i ask cuz i get the same thing as you (i have a mostly stock 88 GT vert. still SD). right off idle with light throttle, i get some ping (from 1K - 1.3K). sometimes i will take off in 2nd on a downhill, and i get the pinging too. i think it is heat/timing related - this does not happen in winter. i just backed my base timing back down to 12* (from 15, which i run in winter). it is 100* here now. i might need to go back to 11 or 10*.

i too have not figured out exactly what happens.....it is like lugging a car, but im not luggin in 1st gear. i have wondered if im running lean off idle for some reason. if i take off at 1300 rpm or more, no problems. weird. just thought i would toss that out in case it is similar to your experiences.

good luck.

 

[blkstangman88]

my timing is at 13 and there is absolutely no pinging, its just like a hesitaion, like its not getting enough gas or somthings holding it back, untill i put the pedel to the floor. when i put the pedel to the floor its like back fires 1 or 2 times then its like i have full power and the car takes off and its fine. and yes the fuel system is all stock.

 

[HISSIN50]

my timing is at 13 and there is absolutely no pinging, its just like a hesitaion, like its not getting enough gas or somthings holding it back, untill i put the pedel to the floor. when i put the pedel to the floor its like back fires 1 or 2 times then its like i have full power and the car takes off and its fine. and yes the fuel system is all stock.

oops, i guess i did not catch that (when you said predetonating, i thought you meant preignition or detonation [often incorrectly used interchangably]). my bad.

also, i missed DMAN's post the first time. sounds like we (he and i) both have the same sorta deal.....and backing timing off (or perhaps increasing octane) helps it out (for both of us). cool.