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

[gilly]

I do have a haynes manual, I'll have to look into what's involved in testing the CPS.

I'd be interested to know what a SOHC 4.0 timing should read at idle and under load though.

Seriously considering an X3 Flash tuner for it in the new year. I'd love it for the transmission shift control alone!

 

[robertoa1a]

I haven't seen a negative advance (retard) timing on anything. There is a text book method of dialing the cam sensor in on the push-rod (non-overhead cam engines) using a volt meter. Perhaps there is dieseling (preignition) causing knock. I will have to take a back seat on this thread and learn something from you OHV folks.

Have patience, a lot of the posters are from the US and are in a different time zone.

 

[Danson]

had the same problem, replaced pcv valve and fuel filter and fixed it all

 

[2000StreetRod]

. . . I'd be interested to know what a SOHC 4.0 timing should read at idle and under load though.

Seriously considering an X3 Flash tuner for it in the new year. I'd love it for the transmission shift control alone!

I looked at some of my recorded data for past drives on 87 octane unleaded regular gasoline. The ignition advance is around +10 degrees at idle. At moderate throttle and accellerating the ignition advance is around +20 degrees. At 3,000 rpm, light throttle and decelerating the ignition advance is +42 degrees.

I agree that your fuel trims are probably bogus. Too bad, they would be very helpful. If they were as you listed you would get DTCs set. The STFTs can change rapidly. I've seen them change as much as 25% in 0.7 seconds.

A rapid increase from light throttle to moderate or wide open throttle should cause the PCM to switch from closed loop to open loop. When you "mash" the accelerator the PCM recognizes that by the change in the TPS voltage and adds fuel to keep the engine from stumbling until the airflow increases enough that the MAF sensor reports it. If you have not performed the TPS Test Procedure I suggest that you do. Be especially watchful for erratic output voltages for rapid throttle changes which are easier to see with an analog meter than a digital meter.

If you purchase an X3 from Henson Performance James will point out things that are abnormal with your engine when he analyzes your data logs. I have started a thread documenting my in process custom tune: Custom tune by Henson

I hope that you have someone else driving your vehicle while you observe PCM parameters with your scanner. Don't kill yourself trying to fix your performance problem!

 

[2000StreetRod]

Read the ECT

Read the engine coolant temperature (ECT) with your scanner. It may have been damaged when the front timing chain tensioner was replaced. It should be about 195 degrees F.

You stated in an earlier post that disconnecting the wire loom to the MAF sensor made no difference. Was that while driving or just in Park? Driving without the MAF sensor connected should have made a big difference. Also, there should have been hesitation upon rapid throttle increase even in Park. With your scanner read the MAF sensor airflow at idle, 1000, 2000 and 3000 rpm while in Park.

With the ignition On and the engine Off use your scanner to read absolute throttle position at closed throttle, WOT and then slowly decreasing back to closed looking for erratic readings.

 

[2000StreetRod]

Fuel pressure?

Did you ever check your fuel pressure at idle (with & without vacuum), and at 3000 rpm with vacuum? The fuel pressure at 3000 rpm should be greater than at idle with vacuum. The fuel pressure at idle without vacuum should be greater than the fuel pressure at 3000 rpm with vacuum.

Here's what might be happening.
When the accelerator is rapidly depressed the PCM should immediately increase the mixture richness to keep the engine from stumbling. This requires good fuel pressure at a high flow. If the fuel pressure can't provide the flow demand the engine power drops. Since the engine power is low and can't satisfy the commanded throttle increase the PCM retards the spark advance.

 

[gilly]

Thanks for the replies once again guys. Off to work I'll look into all this later today

cheers
gilly

 

[robertoa1a]

Your inteligence level is above the norm. Geniouses have a hard time functioning. Par for the course! Now fix the damn car!

 

[storlied]

Have you tried seafoaming your engine? Carbon buildup can cause all kinds of issues.

 

[gilly]

Merry Christmas all!

Thanks once again for the suggestions. CPS was replaced when the timing chains were replaced - if this needs to be dialled in, hey it's possible that it's not quite right.

2000streetrod - my timing readings are very different from idle all the way. I run usually around 95 octane fuel through the X all the time, I can't see that having a huge affect on things though.

As for the TPS, I've swapped that over for a spare I have, both give similar readings, and I can check via OBDII, even with just the ignition on but not running. It's a smooth signal all the way. Foot off throttle it reads about 19% and WOT is 93%. Nothing erratic in between.

Coolant Temp Sensor is accurate - checked.

Fuel Pressure - What's holding me back from testing this is the Schrader valve. It's near impossible to get your hands on a fuel pressure gauge in australia with this valve. I've got a mate about 1hr's drive away with one, hoping to get the time to get over there to test this in the near future, but there's no fuel smell from the fuel reg, and I've replaced the fuel pump and filter, so I'd doubt that there's problems here. Also once it gets over this hesitation, there's plenty of power on tap all the way to 5500rpm, certainly doesn't feel like it's starving.

MAF readings in Park - I'd be interested to compare these with another SOHC. Won't get time to get these today being Xmas, but will report back when I can. Driving without the MAF - I didn't do this for long, just around the block. Just got in and mashed it to WOT and it hesitated at the same point so I guessed that it wasn't to blame. More testing required?

I'm thinking of pulling a plug or two to check the gaps - would this possibly affect it?

 

[gilly]

Have you tried seafoaming your engine? Carbon buildup can cause all kinds of issues.

Don't even know if I can get Seafoam in australia? I'll look into it but again when the timing chain was done and engine was out, everything was cleaned and inspected. Rocker and sump covers etc were even stripped cleaned and resprayed. The internals looked as new apparently. It's been babied and fed clean high octane fuel most of it's life too.

 

[gilly]

had the same problem, replaced pcv valve and fuel filter and fixed it all

Fuel filter I could understand (and I've replaced), but PCV causing this - I don't see how?

 

[2000StreetRod]

Fuel pressure

. . .

2000streetrod - my timing readings are very different from idle all the way. I run usually around 95 octane fuel through the X all the time, I can't see that having a huge affect on things though. . . .

Fuel Pressure - What's holding me back from testing this is the Schrader valve. It's near impossible to get your hands on a fuel pressure gauge in australia with this valve. I've got a mate about 1hr's drive away with one, hoping to get the time to get over there to test this in the near future, but there's no fuel smell from the fuel reg, and I've replaced the fuel pump and filter, so I'd doubt that there's problems here. Also once it gets over this hesitation, there's plenty of power on tap all the way to 5500rpm, certainly doesn't feel like it's starving. . . .

I'm thinking of pulling a plug or two to check the gaps - would this possibly affect it?

Based on what you've posted to date I'm guessing either a fuel or ignition cause. You could have some bad gas or accumulation in your tank. I agree that 95 octane should not cause incorrect timing. Your timing is more retarded than mine at WOT on 87 octane pulling an uphill grade from standing start in 2nd gear. Fuel problems frequently cause hesitation. Even though you have replaced the filter and pump you could have bad electrical connections or a faulty pump. If you can't get a Schrader valve compatible gauge you can bleed off the pressure, remove the Schrader valve, and clamp a fuel hose on the tube to measure the pressure. It's actually a more reliable way to measure the pressure.

It's always helpful to "read" the plugs.

 

[gilly]

Thanks for the suggestions. I doubt bad fuel is to blame. I've put a plenty of tanks through with no change. The inside of the tank was still immaculate when I changed the pump over too. I'll get the pressure tested and let you know how I go there.

I'm thinking crank angle sensor could have an affect on timing? Also want to dig deeper into the knock sensor side of things. I might disconnect it temporarily and see if the timing changes wildly.

 

[2000StreetRod]

Knock sensor

Two days ago I did a WOT pull on discount 87 octane unleaded regular from 2500 rpm to 5850 rpm in 2nd gear uphill all the way. The knock sensor retard varied from +0.5 degrees to -1.75 degrees. The spark advance started at +18.5 degrees, dropped to +15.75 degrees at 3500 rpm after the torque converter locked and then climbed in small steps back up to +20 degrees at 5850 rpm. As soon as I shifted to drive and let off the accelerator the spark advance jumped to +40.5 degrees at 5400 rpm. If you disconnect the knock sensor avoid WOT in case of detonation.

As I recall the crankshaft position sensor (CKP) fires at 60 degrees before top dead center on #1 and #5 cylinders and the PCM computes timing positions from that. I doubt that your CKP is bad since your engine pulls good at WOT.

I wish your STFTs were available because I suspect your mixture is lean during the transition to open loop.

Here's an unlikely possibility. The camshaft position sensor (CMP) is bad and the PCM is firing the injectors at the wrong time creating a "stale" fuel charge in the intake ports. The effect is more pronounced when the accelerator is quickly depressed and the PCM increases pulse width based on the TPS voltage increase. You've already eliminated the TPS as a fault candidate by testing and replacement. There is no timing adjustment for the SOHC CMP but your Hayne's manual describes how to test it with your meter.

I still think a brief fuel pressure drop is the most probable source of the problem.

 

[2000StreetRod]

What brand plugs?

What brand of plugs did you install and how long ago did you install them? Several forum members have had early failure (3,000 miles) with Bosch platinum plugs.

 

[gilly]

OK I feel like a bit of an idiot but I thought the knock sensor was actually located right where the CMP is until I double checked the Haynes manual, therefore the other day when I said I replaced the knock sensor, it was actually another CMP I pulled from a wreck - no difference there, and I can't locate the bloody knock sensor!? I know where it should be. RHS head near intake manifold? Anyway that's the CMP off the list, and the possibility of a knock sensor failure back on the list.


Sitting behind the wheel in the drive right now - thought I'd post some data.

At idle in park:
RPM: ~650
Timing Advance: 16-17
MAF flow rate: 4.6g/s

1500rpm in P
Timing adv: 20
MAF: 11.2g/s

3000rpm in P:
Timing adv: 20
MAF: 20.2g/s

On acceleration up hill:
Timing adv: Drops to -4 at point of hesitation then jumps back up to 11~15 once it's past it's hiccup point and pulls strong at that all the way through.

Here's the part I don't understand:
If I accelerate gently to keep it in closed loop past 3000rpm and then mash it to WOT it'll pull without a hesitation, perfectly. Surely if it's fuel related it would still starve momentarily with the change to open loop in this case?

My brother did the plugs back when the car was his. It's hardly done 3000miles since though and has hesitated from before they were installed, right through. I remember he said that they were platinums but I'm not sure which brand. They are definitely gapped correctly though.

 

[2000StreetRod]

Knock sensor location

The photo below of a knock sensor is from RockAuto's website.

It is located on top of the passenger side head under the intake manifolds and the injector wiring harness. You may be able to get to the connector to unplug it but it's a big job to change it. I wouldn't bother unless I was sure it is defective.

I'll look at your timing and airflow data and get back to you.

 

[2000StreetRod]

Sensor location photo

The arrow in the photo below identifies the knock sensor mounted on the top of the passenger head.

As you can see, the manifolds would have to come off to get to the sensor.

Your posted spark advance and airflow seem fine at those engine speeds.

How about checking something on the VIS? You said you replaced the inlet air control valve (electric solenoid) but have you checked the vacuum motor operation? The vacuum motor is just aft of the IAC valve. There is a rod/lever that connects the vacuum motor to the intake manifold runner control (IMRC). At around 2500 rpm the rod should start to move the IMRC. At 3000 rpm the rod movement should stop. Some forum members have found the rod disconnected. One found the rod laying on top of the heads. If the rod is connected but doesn't move smoothly between 2500 and 3000 rpm try disconnecting and plugging the vacuum line to the vacuum motor. If the hesitation stays the same then I think you've found the problem. The air inlet control valve could be bad, the vacuum hose to the vacuum motor could be leaking, the vacuum motor diaphragm could be ruptured or the intake manifold runners could be stuck from carbon buildup. You can check the air inlet control valve with a vacuum gauge.

 

[gilly]

Thanks for the pic and checking my data.

The sensor sure is buried deep in there. If I could find the other end and disconnect the loom I suppose I could still test it to a degree though. Does that cable and loom run towards the front or back? I'm guessing going by the pic that it's hooked up at the front?

As for the VIS - I'm sure it's working - It was jumping off months ago but that was fixed. I just took it for a drive and stuck a bit of blu-tak to the extremes of where the rod would move to, when I popped the hood after the drive there was an indent in the blu-tak where the rod had pulled to. So that's all working and off the list.