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Ask Ted Ritter Your Own Question
Ted Ritter
Ted Ritter, Toyota Master Tech
Category: Toyota
Satisfied Customers: 4182
Experience:  Toyota Master Tech, ASE Certified
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97 toyota tacoma 2.4l hesitates on acceleration

Customer Question

97 toyota tacoma 2.4l hesitates on acceleration
Submitted: 8 months ago.
Category: Toyota
Expert:  Matt replied 8 months ago.

Hello

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.

As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.

Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.

Work your way through each possible joint one at a time and you should find it. I use a slightly flattenedpiece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Customer: replied 8 months ago.
I have already used carb fluid and sprayed around the hose. Found two hose leaks that were repaired. Hesitation still remains. Customer had knk sensor, fuel filter, fuel pressure regulator replaced by another shop. I have replaced the upstream oxygen sensor, checked fuel psi at 38, ran scanner and noted that fuel trim is good at idle. Tried running with another ecm, maf and same result. Adjusted tps, monitored tps using scanner and analog multi-meter and it checks as good. Checked spark plugs and wires, ignition coil for carbon trails, arcing etc. None found. Checked that egr is closed and catalytic is free. Any other suggestions?
Expert:  Matt replied 8 months ago.

Hello

Ok in that case try a check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

I'd also check the condition of the engine ground connections, these all have to be tight and clean

the main battery one is obviously important but also check the small wires that run out of the wiring loom and are secured to the engine with ring terminals

also when you have the monitor running how does the ECT and IAT readings look?

steady and stable ?

As if not then its possible a duff signal form either of these could make the engine hesitate

Customer: replied 8 months ago.
I had already replace the exhaust rings as they were leaking. I have checked and clean all grounds on the body and battery. I have check the vacuum and it is at 22 at idle. Fuel trim readings good at idle. Depress acclelerator and timing goes to 10 degrees after tdc and engine backfires.
Expert:  Matt replied 8 months ago.

Hello

Ok I'd next try pulling the distributor cap off the engine and inspect it for cracks and damage, do the same for the rotor arm

I don't think that the timing is adjustable on the distributor body but yo ucan check it

At the diagnostic connector under the hood, install a jumper wire between the TE1 and the E1 terminals. Restart the engine and check the base timing. It should be 3 to 7° BTDC. Remove the jumper wire and check the controlled timing. It should move between 7°-18° BTDC.

if its moving more than this then is a sign of loose timing chain.

Customer: replied 8 months ago.
The distributor is not adjustable, With the jumper in place and at idle the timing is at 5Deg. Very stable. The control timing is within range as well. The rotor has slight pitting and will be replaced tomorrow.
Expert:  Matt replied 8 months ago.

Hmm

not that then

is the carbon brush in the distributor cap in good condition ?

Customer: replied 8 months ago.
Cap seems to be in excellent condition.
Expert:  Matt replied 8 months ago.

Ok

in that case I'd suggest doing a compression test on all cylinders to confirm that the engine health is good, as they should all be within 10-15 psi of each other

Customer: replied 8 months ago.
Replaced the rotor. No change. Tried with camshaft sensor disconnected. No change. Really has me baffled.
Replaced spark plugs. No change. Closed off egr valve. No change. Tried removing studs from distributor and turning distributor. No change. Leaves me to believe that the ecm is doing all the changes based on the crankshaft sensor input. Tried varying the tps and maf signals using a signal generator, no change
Expert:  Matt replied 8 months ago.

Hello

Ok I'm going to opt out and get another expert involved, whom hopefully has direct expierence with this issue

good luck

Expert:  Ted Ritter replied 8 months ago.

Hello, I have read all the questions and comments posted on this thread and all the things that were tried already but there are two things I see that might be the problem, if you've already tried this please disregard them but it's all I have for you:

try cleaning the throttle body, get all the carbon deposits out and make sure the gasket is in plsce

the expert that opted out may have been on to something about the ECT sensor, it's located at the end of the cylinder head near the firewall. good luck and please let me know if this helped at all, thanks.

Customer: replied 8 months ago.
I have checked the ect and it is reading properly, tried running with it disconnected but no difference. I have further checked valve clearance and timing marks on chain and gear. I will check the crankshaft sensor later today. Reluctor does seem good. Other than that I am at wits end.
Customer: replied 8 months ago.
I did clean throttle body and plate, no difference
Expert:  Ted Ritter replied 8 months ago.

thanks for the reply, it's possible the it has a bad ECM.

Customer: replied 8 months ago.
Thought about that so I tried another ecm and maf. The timing retards to 10 deg and stays stuck until throttle is released. I have removed the ckp tested resistance and inspected it and it is good.
Expert:  Ted Ritter replied 8 months ago.

Thanks for the reply but I don't have any other ways to go on this except that maybe there is a wiring harness problem between the engine sensors and the ECM.