Help! Hesitation right off of idle.

[Ryeguy]

My L98 Vette hesitates for a split second only when throttling up off of idle.. It wont do it if I get on the gas at any other rpm, just idle. Any ideas? Plugs, wires, air filter are all new. Timing was set recently , MAF is clean, TPS is set, All injectors are around 16.5-17 ohms. Static fuel pressure is ~40.

I have another issue, which might coincide with this one, if I've been driving it and park it for a short amount of time then try to fire it up again it cranks and cranks sometimes then sputters and finally will start.. but it's only when the weather gets warm.. above 60.

Any ideas? Thanks!
Ryan

 

[surfer93]

Play in the throttle cable or maybe the butterflies are sticking.

 

[TedC]

Leaking injectors? You have a GM service manual? If you are having long cranks after hot soak, say 5-10 minutes after shutoff with engine hot, you may have leaking injectors. I had this issue on my 1988. Raw fuel leaks from the injectors because they are under pressure, messing up the combustion mixture when you go to restart. Check fuel rail pressure again, this time after a 5 minute hot soak. Turn ignition to on (do not start). You should hear the pump realy click and the pressure build quickly to 40 PSI. Make sure it holds steady. If the injectors are leaking the pressure will not hold.

 

[boomdriver]

what about the distributor & cap?

Sometimes the ignition module gets tired and weak. That HAS to be delco.
Spark is harder to create and ignite the mixture when the cyl is suddenly loaded, as in standing on it from a idle to WOT. If the spark is not hot enough or advancing, a hesitation is what you'll get.
Many of these so called "+somethin" or "split-this" plugs will also do that. The gimmick plug mfg do not want you to know that they do not have a heat range for most makes and models, just one plug that fits the thread size and gets sold to fit ANY motor with that size spark plug hole. The same split-fired plug thats listed for your motor is also listed for golf carts, delivery trucks..etc

Plain 'ol AC Delco plugs work great. I think it was some +4 Platinum junk that created a mis as well as a hestitation in my motor.

 

[aklim]

Got a scantool or a DVOM? I would like to see the output to the ECM. I would rather have a scantool to see what voltage the TPS starts out at and when you SLOWLY increase the pedal, is it smooth or is there a burp. Best done with the engine off.

 

[Ryeguy]

what about the distributor & cap?

Sometimes the ignition module gets tired and weak. That HAS to be delco.
Spark is harder to create and ignite the mixture when the cyl is suddenly loaded, as in standing on it from a idle to WOT. If the spark is not hot enough or advancing, a hesitation is what you'll get.
Many of these so called "+somethin" or "split-this" plugs will also do that. The gimmick plug mfg do not want you to know that they do not have a heat range for most makes and models, just one plug that fits the thread size and gets sold to fit ANY motor with that size spark plug hole. The same split-fired plug thats listed for your motor is also listed for golf carts, delivery trucks..etc

Plain 'ol AC Delco plugs work great. I think it was some +4 Platinum junk that created a mis as well as a hestitation in my motor.

Thanks for the input, but it hesitates even if I get on the gas like 1/8th throttle and not just WOT.

I'm not sure what plugs are in the car, I didn't change them, previous owner did. I will try and check that soon. I'll check the cap and rotor asap as well. How will I know if its Delco? Thanks.

 

[Ryeguy]

Got a scantool or a DVOM? I would like to see the output to the ECM. I would rather have a scantool to see what voltage the TPS starts out at and when you SLOWLY increase the pedal, is it smooth or is there a burp. Best done with the engine off.

I have a multimeter. How would I do this? Thanks

 

[aklim]

I have a multimeter. How would I do this? Thanks

On the TPS, there are 3 wires. One is ground, the other is 5V reference and the 3rd is the signal to the ECM

Tech Tips: Adjusting the GM TPS (Throttle Positioning Sensor)

 

[boomdriver]

Like Asklin is saying,
you might have a flat spot on the TPS. That should show a smooth range from near 0 to the top range with no dead spots. Most Delco parts have a tag on them. The reason I mention it is because so many of the aftermarket parts are made generic to fit other applications. Delco electronics are made "vette only" as OEM parts. I have had various parts store ignition parts and every one of them ended up creating more of a problem than it solved. Even spark plug wires...A nice pretty set of "performance" wires caused my motor to knock & ping like a marble in a beer can. I installed the old delco wires and solved the problem once I had gone thru everything else.

 

[Ryeguy]

Like Asklin is saying,
you might have a flat spot on the TPS. That should show a smooth range from near 0 to the top range with no dead spots. Most Delco parts have a tag on them. The reason I mention it is because so many of the aftermarket parts are made generic to fit other applications. Delco electronics are made "vette only" as OEM parts. I have had various parts store ignition parts and every one of them ended up creating more of a problem than it solved. Even spark plug wires...A nice pretty set of "performance" wires caused my motor to knock & ping like a marble in a beer can. I installed the old delco wires and solved the problem once I had gone thru everything else.

Thanks, I will double check some of those things and report back.

I'm also having a weird start up issue. When the weather is warm my car starts harder, what would be causing this?

 

[boomdriver]

well, if its all the time when ambient temp is warmer and the engine is not warm yet, might be as simple as the warmer outside air being thinner than cold air. Its not much but we know the colder the air the better they run.

 

[Paul Higg]

I had a 94 so I’m not to familiar with the L98 but does it have open/closed loop?

 

[aklim]

I had a 94 so I’m not to familiar with the L98 but does it have open/closed loop?

It does. Before it gets to temp, it is in open loop.

 

[Paul Higg]

Right. So I was wondering if it is always in open loop. My 94 ran like crap in open loop in the winter.

It could be a TPS. I replaced mine with an Eckler’s adjustable and it ran perfectly after that. Easy to install and adjust and even comes with a jumper harness to tweak to the perfect voltage.

So far, from everything I just re-read it would seem to be a TPS.

You should have codes though and it’ll run REAL rough and if you have an automatic it’ll shift hard.

That engine seems to have been BUILT around that TPS. I thought it was the weakest link in the whole engine. I opened my old one up and it’s just a rolled piece of kapton with a couple of wires in it. Basically a variable resistor.

 

[aklim]

Right. So I was wondering if it is always in open loop. My 94 ran like crap in open loop in the winter.

It could be a TPS. I replaced mine with an Eckler’s adjustable and it ran perfectly after that. Easy to install and adjust and even comes with a jumper harness to tweak to the perfect voltage.

So far, from everything I just re-read it would seem to be a TPS.

You should have codes though and it’ll run REAL rough and if you have an automatic it’ll shift hard.

That engine seems to have been BUILT around that TPS. I thought it was the weakest link in the whole engine. I opened my old one up and it’s just a rolled piece of kapton with a couple of wires in it. Basically a variable resistor.

If anything, I would suspect that the O2 sensor is dropping in and out of closed loop

 

[Paul Higg]

Yup, that’s why I asked if it was throwing codes. Any codes?

 

[Ryeguy]

Yup, that’s why I asked if it was throwing codes. Any codes?

You guys are describing some of the symptoms I'm experiencing. I had a code once, but I shut the car off and started it back up and it never came back. The car does shift hard both into gear and while driving. The gas mileage is slightly below average. The fans come on way early. It starts hard in warm weather, then idles rough until something kicks the idle up. Sometimes it pulses a little bit when I'm sitting in traffic.

I need to check that TPS out I suppose.

 

[Paul Higg]

Real easy to tell...if you have an automatic a bad TPS will not allow it to hold cruise.

But, Rye, it sounds like you definitely have a bad TPS. Been there. :bash

 

[aklim]

Real easy to tell...if you have an automatic a bad TPS will not allow it to hold cruise.

But, Rye, it sounds like you definitely have a bad TPS. Been there. :bash

Why? All the TPS does is send a signal back to the ECM. It takes 5V and based off the rheostat in the unit, the voltage is different. So if you have a bad spot here but not there and the TPS is there, the cruise can still work. At least that is what I would think

 

[Paul Higg]

Look at the wiring diagram in the FSM. It’s all there. Besides, I learned this from experience. ad
PLUS, cruise varies the TPS so it could easily hit the bad spot.