I have a bit of an odd engine problem.. The power of my engine "peaks" at around half-throttle. If I only push down on the gas half-way, the engine doesn't have any problem revving up really high really quickly. However, if I stomp it, it seems to almost stutter, especially when the engine's cold. I don't think it's a fuel delivery thing, since I can rev up to redline with no problem, and I don't think it's a restriction in the exhaust for the same reasons. (Of course, when I'm reving it to redline I'm in neutral, so I know things are a bit different...)

But anyways, could an out-of-whack IAC/TPS cause problems like this? I was thinking maybe the TPS had something to do with the timing advance and if it was off, it would advance the timing too much for WOT and screw things up. But I really have no idea how that works.



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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else

 

At half throttle, you are probably still in closed loop, while in WOT you are in open loop. What do your spark plugs look like?

Sounds like your AF ratio is off. Most likely too rich because if you were lean, you would most likely be hearing pinging.

Have someone follow you and boot it, if they see black smoke (and your plugs are "sooty") you are too rich and you have to get less fuel to your engine.

 

Hmmmm, it could be a fuel filter, if you haven't changed it in a while. How long has it been? May as well replace it if it's been a long time just to rule that out.

 

I changed the sparkplugs about a thousand miles ago, and they were brown. No buildup on them really, but they did have rounded edges. Nothing unsual there, though.


And umm... I've never changed the fuel filter. I bought the car about 6 months ago and just haven't gotten around to checking that. This weekend a few friends and I plan on working on our cars, so I might do it then, if this freezing rain goes away.

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else

 

Glenn, isn't it true that the vehicle will run in open loop until it reaches operating temperature, at which point it goes into closed loop? Throttle position really has nothing to do with it.

I agree with Mike, change the fuel filter. While you're at the parts store, grab a bottle of Heet, too, and then fill the tank with fresh gas. Then, clear the computer of any codes (see the tech articles for how to) and give her a drive around. If she still acts up, check for any set codes.

When my TPS went out the engine would react to accelerator input just fine. When it was cold, though, it would rev itself at stops (in gear with my foot firmly on the brakes). The TPS was only $30. I doubt you have an IAC problem.

 

Hey man check your cat. The same thing happened to may car a while back. I coudln't figure out what was wrong. i could floor it doing 40 and my car would start going slower. But in nutrual it reved fine. take the exhuast off the back of your cat and check. See if you cat fell apart and a pice is blocking the whole, This is what happend to me. Hey I figure i got a gutted cat out of the deal though.
Jason
ps. Then again I am an idiot so maybe you shouldn't listen to me

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1988 Pontiac Trans Am GTA
350 tpi
BEST ET 15.4

 

Da, actually Throttle position is VERY important for determining WOT. And it really isn't WOT (no such thing as the eprom is concerned. It is actually PE or Power Enrichment Mode which is entirely controlled by throttle position. There is a table in the eprom that allows you to specify at certain rpm/% throttle position when you want PE to engage. When PE is engaged, it adds extra fuel and extra spark, again based on specific tables.

All those people that are "hell bent" on getting 4.5 volts an their TPS to get WOT all wasting their time. It is controlled in the eprom. I can go into PE WAY before 4.5 volts even 2.5 volts if I wish.

Also PE comes into play on "overly high coolant temperatures". This is to avoid detonation and avoid stalling.

 

Glenn, I don't want to sound like a know it all, but I'm still not with you. WOT = Wide Open Throttle, which, I'll grant you, does not necessarily apply to a fuel injected engine. But, open loop is the computer running the vehicle without certain input, such as the O2 sensor. Once the engine is at operating temperature it goes into closed loop, which utilizes all of the sensors.

Your statement above that says "throttle position is VERY important for determining WOT", is, of course, correct when one remembers that WOT is wide open throttle. So, I think you and I are talking about two different things. Throttle position does not affect open/closed loop. Engine temperature does not affect WOT.

Does anybody else see where I'm coming from?

 

I thought that at WOT the ecm goes into open loop ignoring o2 sensor and bases its calculations on 'limp home' or PROM info only. Thats why fuel pressure only really effects wot and cold operation because the o2 re-adjusts it. At least this is what I was told somewhere.

 

No Da91, I am trying to explain EXACTLY the eprom does. Forget everything you ever thought you knew about WOT...it is wrong. To a "user" WOT is stomping on the gas peddle. To the ecm, this means nothing.

It goes by load and rpm and basd on pre-set throttle position factors within tables.

If you can grasp then, then I will continue with the lesson. When the throttle position hits this pre-set amount it goes into Power Enrichment mode. A special "closed loops" that ignores the O2 sensor reading and relys on tables. If it went into "open loop", your TC wouldn't lock.

But other than the PE table and few other constants, the Throttle Position has absolutely nothing to do in the calculation of the injector pulse width or the spark tables, the ultimate part the ecm controls.

Trust me, I can make the eprom go into PE Mode at any %TPS within the eprom; it does not need to see 100% which is what you would consider WOT. The biggest factor the Throttle has AFTER PE has been invoked is the amount of air allowed in, and since it is not controlled by the O2 it has very little to do as the TB Blade does not need to be opened 100% to see max load.

I do this all the time when I am "testing" my VE tables. I purposely set the %TPS to an extremely high value to avoid PE from kicking in and remaining in normal closed loop so I can get my BLM/INT. Going up a hill, I can hit max load (minimum vacuum) at %TPS under 50%.

But, I cannot explain ALL of this to you as it would take a book and I am not prepared to do right now.

Goto the DIY_PROM board and all these things will get discussed over the next few weeks. Right now we are discussing the VE table on "P730: Intro".

Trust me, once you get into eprom burning, your entire way of thinking of the engine changes. It has too, or you'll f'up and get poor results (and frustrated).

 

Glenn, I have an AAS Degree in Vehicle Maintenance and an ASE Master Certification; I'm not stupid. It's apparent that you and I are on entirely different pages. It's important to me for things to be crystal clear in my mind. Not all computer controlled systems operate the same, so I'll do us both a favor and grab a book and get a quick refresher on this particular system.

On a personal note, why be so quick to sound like a smart a$$?

 

DaBird, Glenn, et al,

WOT (Wide Open Throttle) is not as significant to ECM mode as some might understand.

As DaBird implys, the engine will start in open loop mode. As soon as the coolant temperature is above 104°F and the oxygen sensor signal is varying, the ECM will attempt to enter closed loop. It will remain in closed loop until the temperature drops, the O2 stops varying, or a major DTC is set.

And Joel is partially correct as well. A relatively high throttle angle will cause an open loop mode, but the mode is Power Enrichment, not Backup Fuel Mode (commonly called "Limp Home Mode"). The PE mode is technically an open loop because the ECM essentially ignores the oxygen sensor input for fuel control.

And, as Glenn details, the percent of throttle position also determines the mode. An engine at closed loop will enter the PE form of an open loop when the throttle is opened beyond a specified figure. The stock PROM programming on most ThirdGens has this point defined at 80% of maximum throttle, but this can be changed easily by anyone who chooses to edit the fuel and spark offsets and tables.

So you are all correct, in a manner of speaking, but no one is correct to flame the other because of a misunderstanding, nissed percetion, or semantics. Really, you are each stating important functions of the same system.

And unfortunately, not much of this helps Enkil, who probably has weak/late spark, lean mixture, or some restriction. That's just my guess.

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Later,
Vader
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"No matter how hard you try you can't stop us now..."
Adobe Acrobat Reader 4.0

 

Da91bird,
Obviously you are fairly new to this board and don't know Glenn very well.
If you stick around you'll find that Glenn is just plain smart when it comes to TPI ECM and the programming that makes them tick.
He is always quick to help people with their questions and problems and he knows his stuff.
I believe you could learn alot from listening and following his advice.
Dave

 

I had already gone to bed, but was just eaten up enough by this thing to come back and try again. Everything that Vader said, is what I came back to say; he's one diplomatic dude. Specifically, I felt bad for poor Enkil who just wanted his question answered.

I know I could learn from Glenn, but not because I'm clueless. That's just the way that all of that came across to me. It's true that I'm new to the board, but I'm certainly not new to engines and what makes them tick. Put yourself in my shoes, that's all.

 

Sorry, I have typos in my messages; trying to type very fast while being very tired (long day at work). My apologies. It came across badly.

All I am saying is that you have to "think like the ecm and forget what we as drivers/gearheads see"; it makes understanding the internals much easier.

As for a book, I wish there was as book on all this. The problem is that the book is being written as we speak. There are things in the eprom that I am sure that only a few GM engineers truly know how they work.

For example, to make Highway Mode (and Highway Mode Spark Advance) work properly, you have to go beyond the typical "editting of a variable/constant/or table" as we normally do (though you have to do some of that too). But physically get into the "machine code" and "patch" the actual machine instructions in hexidecimal to actually get it to work. Right now, if someone asked me "exactly" what instructions you have to change, I would not tell them as I have just been working on this now and finally got it working with a "shotgun" patch. Now I have to remove each "patch" individually until I find the exact instruction that wasn't working properly that I fixed.

Life would be easier with a "debugger/tracer", but there is none available and I don't have a "benchtest" with "standard' electrical inputs to emulate the operation of an engine to speed up the testing process. Guess I'll have to look into getting one of those.

But a lot of this stuff is quite advanced and it is a bit difficult until you actually "get your feet wet", see the actual tables, test the tables and monitor the effects. Then these explanations make a lot more sense and become much easier to understand. I am definitely not trying to be a smart a$$, elitest, or anything else. Just very tired and trying to give a brief explanation on a complex subject using the eprom's proper terms.

But I do encourage you to start scoping out the DIY PROM Board, read Tim Sifford's (Traxion) article on "Prom burning 101" (link on the top of the PROM Board and begin reading the oldest posts to the most recent.

With that "base of knowledge" (and don't be afraid to read it a couple of times) then these "terms and concepts" will all come together and the light bulb will click on. That is when you realize that virtually anything is possible then and even consider "rewriting the actual machine code" to make an even better eprom.

It just takes time, patience and a LOT of testing.