Hesitation in 95 Tahoe 5.7L HELP!

JeffP · May 17, 2002

Hello,

I have a 1995 Chevrolet Tahoe 5.7L TBI 4wd/4dr, 60,000 miles, mint condition.

I have been experiencing a "hesitation/stumble" when I lightly step on the accelerator. This happens either from a standstill OR while cruising. It happens ONLY when I give LIGHT throttle pressure at the low end rpm range of the gear ( i.e. early into the gear, very light throttle at low rpm ). The car will hesitate & buck slightly unless I step harder on the gas to get it to smooth out.

Associated with this hesitation/stumble is a general loss in power/acceleration/pickup, the truck feels VERY sluggish ( most often when I need to pass or accelerate -it's a DOG!). If I step on the gas in any gear to smooth it out, the acceleration will SLOWLY smooth out ( with not much power ), ending up in a high rpm "SURGE" feeling until the next gear is engaged - WEIRD! Seems like the engine will max out almost in that particular gear as it struggles to shift to the next gear.

This ONLY happens after the engine has reached a certain temperature, it will not behave like this in the morning, while the engine is cold.

I've had the EGR valve replaced, new cap/rotor, platinum plugs & wires, new PCV valve, air filter, and fuel filter.

Before I take it back to a service center ( who will recharge ~$75 for a diagnostic ), I wanted to see if anyone had any ideas or experience with these symptoms.

p.s. On my first diagnostic report, no trouble codes were reported, fuel pressure was in spec. The only thing out of spec was that the EGR valve was opening too early and too far, hence the new EGR valve.)

Thanks in advance for any advice/suggestions you can offer.

Sincerely,

Jeff

Colorado Kid · May 18, 2002

The only things missing from your list are oxygen sensor (cheap) throttle position sensor (not so cheap) and MAP sensor (proably even less cheap), so try them in that order. My money is on the oxygen sensor as the problem only occurs after the truck is warm, which is when it changes to "closed loop" and starts using the oxygen sensor signal to vary the mixture.

JeffP · May 20, 2002

OK,

I replaced the TPS ( $18.95 from AutoZone - it's a WELLS switch/sensor) - No Dice, same problem.

I'm gonna try the O2 sensor, but I've found out that my LATE YEAR production Tahoe ( Nov 1995 ) actually has 2 oxygen sensors, 1 in the engine compartment & 1 right before the catalytic converter. Of course, they're $55.00 each !!

I guess I'll havta replace both, but I'm waiting on some cash next month.

Thanks for the help & tips! Any way these O2 sensors can be tested before I plunk down the cash?

Colorado Kid · May 20, 2002

You got a TBI engine and 2 Oxygen sensors? :but:

Now I'm confused. It's got to be going lean for some reason, but if it has 2 sensors it must be OBD-II, and it should set codes for misfires or bad sensors. But then again I don't think there were any OBD-II TBI motors. :ehh:

Testing sensors: I can't hing of any good way that wont involve a scan tool more expensive than the sensors. YOu caould try swapping them front to rear...assuming only one is bad that could make a difference (I just don't know for sure what the difference would be). It might then throw a code, or it might run better, or it might even do both!

Back in the computer controlled carb days this is how every GM vehicle drove...we called it inverse throttle response...the harder you pushed the worse it ran until you got it wide open and it finally enriched the mixture. They'd run OK till they warmed up and then go right in the toilet. I don't know if they worked at sea-level, but nobody could sort them out at 5,000 feet.

I'm really trying not to send you on a wild goose chase, but this one has me confused! I really was not expecting oxygen sensors to set you back $110.

CMNTMXR81 · May 23, 2002

In 95, OBDII equipped vehicles were required in California. AS such, some still TBI powered non Vortec 350's were around.

Swapping them like Colorado said would be the only way without a scan tool. You said the EGR was replaced, are you sure it is turning on though. The symptoms you mention are reminiscent of an EGR system not turning on which would result in knock, which then the PCM would retard timing until the knock is gone. This would explain why under full throttle, you don't notice this problem as the EGR only turns on in part throttle applications. Otherwise, it's off at WOT.

Providing that fuel delivery is, well, being delivered adequately it sounds like one of two things.

1) Your ECM isn't controlling dwell or "load"

2) You are someway, somehow getting knock.

I'd check to make sure all vacuum lines are securely connect too. This could be the genesis of the above items I listed.

JeffP · May 24, 2002

Ok guys,

Took the Hoe back into the shop for a re-diagnostics. They waived the $80 fee this time since they tried before and replaced my EGR valve without fixing the real problem.

Anyway, they worked on it all day. Checked compression, timing, fuel pressure, vacuum, switches, etc. everything you could check mechanically, they told me. They could not find what is causing the hesitation at low throttle.

They did confirm the problem of hesitation at low throttle and confirmed the fuel was way too lean during the problem -hence the stuttering/hesistation.

They said the next step was to do some pinpoint lab scope type diagnostics and also check the ECM as everything mechanically checks out ok.

Anyway, went to pick her up, started her, and noticed my radio presets were all gone and my clock was off. Hmmmmmmmmmm, no big deal, they must have had to remove the battery negative during their testing.

Now, the problem has completely DISAPPEARED! No more hesitation, or stumbling!!!! Smooth as glass acceleration & more Power!! It's as if I got a brand new engine or someting!!!!!!!!

Could the ECM have been reset when they killed power from the battery? Remember, no trouble codes were ever reported.

The only thing I can think of is that about 4 months ago ( right around when the trouble started ), I put a new radio in my dash. Of course, I disconnected the negative battery terminal. When I re-connected the negative terminal, several ( about 5 ) illumination bulbs in my dash died ( I replaced them, of course ). I was thinking, could some kind of weird voltage surge ( which killed the bulbs ) have scrambled the ECM? This was the first time my battery has been disconnected since I disconnected it during the radio install.

Does anyone think the ECM got re-set now and that's why the problem went away? Any thoughts?

I'm not complaining, just curious to learn how this problem just mysteriously disappeared all of a sudden........

Happy Hoe Driver :-)

tinman · May 25, 2002

That is the cheap tune up I put on my 90 work van ever year.

It seam to help it out to pull the ecm fuse and let the computer reset it self . Sometimes it make a big differance like in your case. :bored:

CMNTMXR81 · May 28, 2002

They probably re-flashed it. It is now in idle re-learn mode. If you go to www.obdII.com you can see the exact procedure for re-learning your PCM during this.

Essentially they set it back to GM's base curves and now it is re-learning your driving style, the way the engine operates under certain conditions, etc, etc.

Colorado Kid · May 28, 2002

And to add to the above, I suppose it is possible that when you spiked it you erased some of the "learned" curves without reseting the ECM...so it didn't know that it didn't remember right. Pulling both leads and touching them together is a sure way to achieve a reset, and isn't a bad idea at all after replacing some of the components you mentioned. We shoulda told you! I think the chances are very good that your troubles are cured for the long run.

JeffP · June 4, 2002

Ok, Problem came back after about 1 week! Arrrrrrrrrrrrgh!

I disconnected the neg. batt cable for 2-3 hrs then re-connected it. Problem got no better or worse. I guess it's not the ecm then? Does that pretty much rule out the ECM?

I guess the shop may have fiddled with something else which caused the problem to go away for a short time? This is driving me NUTS! Any other suggestions? To recap, the following have been replaced so far: plugs, wires, cap, rotor, fuel filter, air filter, pcv valve, throttle position sensor, egr valve. Shop did diagnostics & told me everything mecanically was working. Fuel pressure ok, timing ok, vacuum, etc.

Problem only happens at low throttle pressure and smooths out as you step on it. Slow, smooth takeaways from a stop are horrendous! Drag-strip starts are fine.

I'm going to pull all my hair out over this!

CMNTMXR81 · June 5, 2002

You need to turn the key on to accessory when you remove the negative cable (much the same as pulling the PCM BAT and PCM IGN fuses if your vehicle is equipped as such). You should only need to do it for a minute or two then turn the key off and re-connect the cable.

JeffP · June 5, 2002

Hmmmmmmm, Interesting. No I never had the key turned to accessory when I yanked the neg cable. Won't that do any electrical damage? I always was paranoid about that.

The only fuse I could find in the engine compartment was a 20A fuse labelled ECM. There is no IGN or BAT fuses, only the one ( I have an OBD-I vehicle, I just confirmed by the sticker under the hood ).

Do I need to have the key turned to accy. then in this case, then yank the neg. bat cable as you suggest?

Just to confirm, I'm turning the key far enough to engage the electrical sysstem only right? ( I'll hear the fuel pump whirr ). Then I yank the cable for 2-3 min, turn the key back to off & re-attach? You're certain nothing will get spiked or anything?

Sorry to be so paranoid, just don't want to fry the new $350 Alpine deck I slaved over to get installed.

What will this procedure do, exactly, when you say "reset the ECM"? If my hesistation comes back after the reset, can I pretty much rule out the ECM as the source?

Thanks for the continued help & patience.

Jeff

CMNTMXR81 · June 7, 2002

Yup. You won't do any damage at least to the stock components. For your Alpine system, for piece of mind, it may not hurt to pull the radio fuse before doing this. The reason you have the key on to accesory is to turn the system "live" then once it has a power intuption while on, it resets itself to GM's base curves.

This is a common procedure on GM front wheel drive cars with underhood electrical centers. Turn the key on, remove PCM BAT, PCM IGN fuses...let simmer for a minute or two, shut off ignition, replace fuses PCM IGN, PCM BAT and the PCM will be reset.