98 SOHC - Hesitation driving me crazy! Help needed...

[gilly]

Hi All,

Long time reader, first time poster here from Australia. Very informative forums here - well done to you all!

I've got a 98 Explorer XLT, SOHC V6, auto. It's done 185,000km/115,000 miles and I've recently done the following:

New Timing chain
New Plugs & Leads
New Coil Pack
New Injectors
New Fuel Pump
New intake gaskets
Cleaned MAF and Intake Temp sensor,
Checked readings from MAF, TPS, Intake temp, Coolant Temp etc are all on the money
O2 Sensors appear healthy
No vacuum leaks that I can detect
It's never thrown a code or CEL.

It idles fine, cruises fine. light acceleration is fine, but give it some throttle, or especially WOT going uphill or under load conditions and it hesitates bigtime. It's driving me crazy and I've spent so much money and time on this thing it's a shame that I can't drive it and enjoy it.

I'm hoping that someone can offer a new suggestion to put me out of my misery!

cheers
Gilly

 

[2000StreetRod]

replaced fuel filter?

Have you replaced the fuel filter in the last 30,000 miles? If not, then I'd change it and then add fuel injector cleaner to your fuel tank.

Did you check the fuel pressure after replacing the fuel pump?

Are you using 87 octane or higher fuel? The knock sensor can retard the timing causing loss of power upon acceleration.

 

[gilly]

Have you replaced the fuel filter in the last 30,000 miles? If not, then I'd change it and then add fuel injector cleaner to your fuel tank.

Did you check the fuel pressure after replacing the fuel pump?

Are you using 87 octane or higher fuel? The knock sensor can retard the timing causing loss of power upon acceleration.

Thanks for the reply. Yes sorry forgot to add that - New fuel filter as well. It always runs at least 95 octane, usually 98 octane.

I haven't checked the fuel pressure, I have checked the regulator and it seems fine. I didn't think about the knock sensor retarding the timing either - something to look into.

cheers!

 

[stangman82]

sure timing didn't get bumped off a little with the timing chain swap? That's where i'd start. possible fuel pressure issue. clean the maf while you are at it too

 

[gilly]

Well it's still not right.

Timing has been checked - fine.
I've replaced the knock sensor, even some vacuum lines that were looking a little tired. It's got a new IAC valve too.

I'm really starting to wonder if it's the O2 sensors. 115,000 miles on the original sensors, would they be about dead? It's not throwing any codes to suggest this but I do have poor fuel economy and this nasty hesitation at 3/4 throttle, and around 2500-3000 rpm usually.

Any help or further ideas really appreciated.

 

[storlied]

This might sound stupid.. but could it be drivetrain related?... See how your diff's are doing... check your ujoints also. When my CV joint on my front driveshaft was on it's way out... my truck would "hesitate" on acceleration until I heard the CV joint pop. You may not have any noises.. but it's worth checking...

 

[2000StreetRod]

Egr?

The SOHC should really come alive at 3000 rpm. Do you have the variable induction system (VIS)? In the US it was installed on 1997 and 1998 models. The lower intake manifold runner length is switched at 3000 rpm for better tuning. There's an inlet air control valve (electric solenoid vacuum switch) that activates a vacuum motor located on top of the upper intake manifold just aft of the IAC valve. As carbon accumulates in the lower intake manifold so does the friction opposing the change in intake runners. I think that's why Ford dropped VIS in 1999. A faulty VIS matches your performance symptoms.

You didn't list the air filter. Is it clean?

If your fuel pressure is good at idle and increases at higher RPM then the only thing I can think of is the EGR system. The PCM closes the EGR valve at closed throttle for a smooth idle and at moderate throttle/WOT for better performance. This link may help: EGR System O&T

At moderate throttle/WOT the PCM should go from closed loop (using the O2 sensors for A/F ratio) to open loop (using stored tables). I doubt a bad O2 sensor is causing the hesitation.

Have you checked the compression on the cylinders?

Did your old plugs look normal when you pulled them?

Do you have access to a scanner? For the money you've spent on parts you could have bought an SCT X3 scanner with a custom tune.

 

[gilly]

Thanks for the replies.

CV's seem fine and certainly no 'pop' noise occurring at all.

Air filter is new. Actually I've swapped it to a high flow pod filter at the moment. It's also got a high flow muffer to help it breath a bit better. There's nothing wrong with it's performance above 3000rpm even at WOT it's great. It does have VIS and I've even swapped over the solenoid that you mentioned, but it had no effect on performance. Even disconnected I can feel this hesitation still. I've pulled the vacuum line from the EGR valve and taped the end of the line to see how it would run with EGR disabled - no difference. Pulled the loom from the DPFE - no difference. Pulled the loom from the MAF - no difference.

Plugs all looked identical and were fine.

The hesitation is a distinct drop in RPM, almost feels like the auto is changing to the next gear and then changing back seconds later? When it's not quite at operating temp, the hesitation is less noticeable, and after about 10mins driving it's really bad.

I've even changed the auto in the last few weeks to a box that's done only 60,000kms - so that's not it either!!

I've got an OBDII reader (USB via laptop one) - never thrown a code. Most sensors all read accurately. Only ones I can't read are the O2's and I think that might be my scanner. A garage I took it to read them fine and said they were healthy.

This is the most frustrating car I've ever owned. Any other ideas?

 

[storlied]

Is there anyway you could get a video of this issue?

 

[robertoa1a]

Before spending more money I would get a print out from a diagnostic gadget to check all the values the PCM is reading. Not just codes. Mark the time of the acceleration hesitation on the print out. If one of the values is out of range like the engine temperature or crank shaft position sensor then we have something solid to work with.

Your odometer reading is not high at all. It seem early for new injectors and coil pack. Though, you saved enough money doing everything yourself to have the extra items.

 

[gilly]

True I suppose this is the next step. I can't keep replacing things hoping for the best much longer. Thanks for the input

 

[2000StreetRod]

Time for a scanner

Rats! I was really expecting your problem to be VIS related since the VIS becomes active at 2500 rpm and the runners should be fully switched by 3000 rpm.

I agree with robertoa1a that you need to record some data with a scanner to see what the PCM is reading. Even without the O2 readings you can check fuel trims, engine load, MAF airflow, spark advance vs throttle position and engine speed. I bought a new (but obsolete) Auterra Windows based scanner on ebay and it has helped tremendously in diagnosing problems. My new X3 Power Flash can record several times as many parameters. Can your OBD laptop reader record/display engine parameters?

 

[robertoa1a]

Mine? Yes. I should use it once in a while. I have an older hand held unit that records then you download the data on to crappy software. I have been trying to figure out how to move the data to a spread sheet so I can confuse myself even worse.

 

[joker1457]

Catalytic converter Clogged any takers???

 

[robertoa1a]

115,000 miles? Cats usually last longer. Streetrods timing advance seems plausible.

The OP is in Australia so we provably wont here anything till midnight. It's summer there.

 

[gilly]

Thanks for the replies. I'm not sure how accurately my cheap eBay obdii reader is capturing the data, fuel trim for example seems to stay fixed at -99.999%.

It might be time to take it to a shop for diagnosis. I'll certainly keep you all up to date on it's progress and outcome.

Cheers
gilly

 

[foxy]

fuel

G'Day Mate , Gilly Ive been having similar troubles and I about to replace the fuel pump .and access it through the rear floor instead of dropping the tank . My problem is the pressures .According to the manual it should be 65PSI plus or minus 8PSI .Mine is sitting at 48 and it fluctuates all over the place but rarely goes below 48 . so we'll see what happens .I'm also considering bypassing the regulator in the tank and replace it with an external one that is adjustable with the return tapped into the filler breather pipe. Mate I also have had trouble with the upper and lower manifold gaskets as that will effect your surging and also idling Cheers Foxy

 

[2000StreetRod]

Engine running very rich

If your short term fuel trim (STFT) is -99.99% then that means the mixture is way rich and the PCM is trying to compensate. Is there a significant (more than 10%) difference between Bank 1 STFT and Bank 2 STFT. Can you read the long term fuel trim (LTFT) for both banks. Record STFT and engine rpm while slowly increasing engine speed to 3000 rpm and then decreasing to 1000 rpm. Then see how the STFTs are changing according to the engine speed.

Have you performed the MAF Sensor Test Procedure?

 

[gilly]

Well as soon as I got home I hooked up the Laptop to the X and went for a drive. I've picked up on a few things based on the real time data that I didn't notice before. Unfortunately I can't capture the data so I just have to observe as I drive.

1. The hesitation is probably linked to the fuel system switching from closed to open loop. It's right at this time, every time that it hesitates. It drives perfect on closed loop.

2. Timing advance drops right into the negatives, usually around -4 at the point of hesitation.

3. Fuel trims are reading -100.00 on both banks short and long term, but I doubt that's an accurate reading, I've seen healthy looking feedback from a high end scanner not long ago. Probably my crappy eBay scanner.

4. MAF readings from what I can tell look accurate, nothing out of the ordinary.

Foxy - I've replaced all of the manifold gaskets AND the fuel pump, no difference here.



When I had the engine out for the timing chain I decided to replace anything that would be getting close to the end of it's life as I really plan to keep the car long term. So yes injectors, coil pack, idler pulley, thermostat housing etc were all replaced as well. It should be running like a new one! Been in the family since new and certainly hasn't had a hard life.

Anyway based on my findings above - any further ideas? What happens at that point when the system switches from closed to open loop??

Thanks for the help all, really appreciated.

 

[robertoa1a]

The timing advance is not advanced at all. Do you have a maintenance manual for checking the health of the cam position sensor? If not, I will look at mine and post the info.