Engine hesitation when cold below 2000rpm (1 Viewer)

[Mfb]

Calling all gurus, I have a '94 FZJ80 aussie spec. I've had a problem that Ive been trying to solve for a while but cant get to the bottom of it. I have a hesitation when accelerating from idle to 2000rpm, the best was to describe it is like theres water in the distributor. After 2000rpm, the car drives normal. When the car warms up, any hesitation becomes less noticeable/goes away.

I've tried to solve the problem by throwing parts at it over a period of time. I've replaced:

• Air Flow Meter
• Fuel Pump,
• Fuel Regulator
• EFI module
• PCV value and grommet
• O2 sensor 2x - Denso and NGK
• injectors
• Distributor Cap - OEM
• Rotor button - OEM
• Ignition Leads - NGK and Bosch
• Coil/ignitor
• Spark Plugs - OEM gapped 0.8mm
• Cleaned Throttle Body (TB)
• Replaced TPS
• Intake hose to TB
• Replace a few vacuum lines, rest seem ok from what I can see
• ...and I'm sure theres more...

I've also replaced the above as part of preventative maintenance.

There are no CEL codes displayed.

This hesitation is noticeable when the timing is at 3-4 degrees, but is not noticeable when timing is advanced to about 6-7 degree.

The car is now on straight petrol, it used to be a dual fuel petrol and LPG. The hesitation was not noticeable when driving on LPG.

I'm not sure what else is left to check - value seats...?

Looking forward to hearing your thoughts and ideas on how to fix this problem.

 

[Mfb]

Any thoughts?

 

[ajax1]

Isn’t aus spec timing suppose to be 7?

Test engine coolant temp sensor or replace, one for ecu not the gauge

 

[Mfb]

Thanks for your reply.

I believe 3 degrees timing is the standard spec for the Aussie spec.

Do you mean wiring harness from the sensor to to ECM?

Could it also be a knock sensor?

 

[Mfb]

Bump

 

[Mfb]

I replaced the coolant temp sensor to the ECM, but that didn’t fix the problem.

i also replaced both knock sensors, that too didn’t fix the problem.

I‘m pulling my hear out trying to fix this problem to no avail, needing some expert advice.

 

[Tachycardic]

Make sure your valve cover bolts are appropriately tight. You can get un-metered air being sucked into the engine. I believe the torque spec is 9ft/lb. Reset the ECU afterwards and see what happens.

If you still have issues, I would just advance the timing to 6 degrees if that fixes the problem. It's not overly aggressive and plenty of people have run 6 degrees of timing without issues.

 

[robnicko]

As you have gone back to Petrol only from dual fuel can you post a pic of the engine bay so we can see the plumbing?
in particular the pcv and other breather on the exhaust side of the valve cover.
It does sound like a vacuum leak what youre describing though

 

[Mfb]

Sorry for the delay in responding. The car has been with the mechanic for about 4 weeks trying to identify this problem and completing other maintenance items.

The mechanic couldn’t find what the problem is, he did a wet compression test which passed, and fuel pressure test which also past.

Problem is still there, no error codes on ECM or TCU, hesitation only happens when cold and during warm up, and only when the accelerator is pressed greater then 1/4. No hesitation on light throttle application.

Once the car has warmed up, the issue is not present.

PS all hose and in there correct places, smoke test also confirmed no vacuum leaks.

Could this be related to the AFM and Air/Fuel mixture? AFM has been cleaned with MAF cleaner, but it hasn’t helped. Followed FSM to test the AFM, AFM resistance is within spec.

Future testing with advancing the timing doesn’t fix the problem, thought it did, but it did not.

 

[CharlieS]

Shot in the dark, but my TPS had an issue caused the same kind of symptoms. Replaced it and the behavior has gone away in all conditions. The bogging was especially prominent in cold weather until the engine warmed up. May be worth rechecking or swapping in a known good unit?

 

[Mfb]

Thanks for the feedback. I’ve recently replace the TPS with a new genuine unit, hasn’t made a difference.

Noting that when the car was on LPG (dual fuel), it ran great when switched to LPG, as soon as it was switched back to Petrol, the hesitation was there.

 

[Trollhole]

Couple things. Make sure the AFM vane isn't sticking when cold. Also did you check the temp sensor inside the AFM and make sure it has the proper values?
Are you sure you have the correct throttle position sensor for that year? Does your ECU year match all the other components in the vehicle considering this is an aussie spec? Have you tested the IAC? Do you have the ability to see real time data? As well check your crank pulley and see if it's actually attached to the rubber isolator. I have seen some spin throwing off what the timing gun says which means you could be a tooth off. Not being to see real time data from OBD I systems suck.

I'm betting the IAC isn't getting the correct signal from the ecu or the wiring to it is faulty or the IAC is faulty.

 

[Mfb]

Thanks Trollhole, good suggestions. The vane isn’t sticking and resistance matches the FSM. I don’t know how to test the AFM temp sensor, but it would make sense to do so, could you please share the procedure?

TPS is genuine matching the VIN and all other parts that have been replace should/do match the car. I haven’t tested the IAC, but my mechanic said he tested it and said it was within spec.

No real time data logging available unfortunately.

 

[robnicko]

long shot but I wonder if there is a vacuum leak where the plenum joins the upper intake manifold at the gasket. As the temp increases the metal expands ever so slightly

did you ever check the valve clearances and at idle is there a slight popping sound coming from the exhaust?

 

[Mfb]

My mechanic said he checked for vacuum leaks with a smoke machine, he found a small leak under the manifold which has been repaired. No other leaks reported.

I intended on getting the clearances checked but my mechanic did a leak down test and compression test, and reported that clearances didn’t need to be done - does this sound right? I’ll need to check for a popping sound from the exhaust, could popping indicate valve clearances?

Furthermore, I tested another AFM specific to my model. The hesitation issue was still there but a lot less noticeable, although my AFM was flow tested today and passed - I’m not sure what to think about that result.

 

[robnicko]

My mechanic said he checked for vacuum leaks with a smoke machine, he found a small leak under the manifold which has been repaired. No other leaks reported.

I intended on getting the clearances checked but my mechanic did a leak down test and compression test, and reported that clearances didn’t need to be done - does this sound right? I’ll need to check for a popping sound from the exhaust, could popping indicate valve clearances?

Furthermore, I tested another AFM specific to my model. The hesitation issue was still there but a lot less noticeable, although my AFM was flow tested today and passed - I’m not sure what to think about that result.

The only way to confirm valve clearances is by removing the valve cover and checking with a feeler gauge. This must be done on a stone cold engine to be 100% accurate.

Popping in the exhaust (at tailpipe) on idle can be an indication of valves not seating correctly and/or a vac leak

When on LPG did you ever get any backfire through the AFM / throttle body as when running on LPG the AFM doesnt really do anything but when running on petrol it does and you did mention that on LPG it ran fine?

 

[Mfb]

That’s what I thought regarding the valve clearances. Last time it was checked was about 50,000 kms ago, where 12 needed to be adjusted.

I’ll check for popping next time I drive.

I’ve only ever had one backfire during start up, AFM and throttlebody wasn’t damaged as best as I could tell. The way the LPG was setup meant it had a relief hole in the intake before the throttlebody as a bypass for the backfire.

Thats also my understanding about the AFM when running on LPG. Although a flow test indicates the AFM is within spec.

Not sure if I already mentioned, the car was running rich, plugs were black and relatively still new. Not sure if this has now been resolved with the vacuum leak under the manifold.

 

[OfficialGamings]

Unfortunately I am no expect to any of your problems (Or Mine :/) Mainly here for updates. But I do have a 1993 FZJ80 JDM, Which seems to be having roughly the same symptoms as yours. My rig used to be fitted with LPG also. But has since had all of the wiring disconnected. (Still haven't had time to pull out the rest of the system). So I just run petrol now. But I haven't just thrown a whole lot of parts towards mine. Mostly just making sure all sensors are in spec (TPS, IAC, VAF) Which they are Also I have checked vacuum leaks with smoke test and cleaned my whole throttle body.
I have previously had a few VAF replacements due to LPG backfires..... But have since gotten 2nd hand parts for it.
Gonna be doing some further testing this weekend, Will mostly likely test value clearances since it was suggested and Possibility a few other things will let you know if I find my issue.

 

[Mfb]

Welcome to the club that nobody wants to be part of…

I tested the IAC resistance which is in spec, 21 ohms cross all test/terminals.

No popping from exhaust.

I’m going to try and get both AFMs flow tested to compare results. I’ve been told that there is a wide range on values within the spec range so it’s worth comparing.

 

[Mfb]

Long shot - I’m going to check for an exhaust leak at the manifold, I’ve been told this could produce a similar problem if there’s a crack in the manifold or perished gasket.

I’ll report back with results.