L6 turbo hesitation that seems to be common

[Guest Perry]

Hi yall,

 

In my dealings with the straight six fuel injected cars I've noticed a lot of them seem to be prone to hesitation, especially in hot weather. Does anyone know what causes this or have you had experience with this? I've had a couple of turbo L28s now that run great most of the time but then hesitate real bad occasionally and it doesn't seem to go away despite some parts swapping, care in buying gas etc.. I have a friend with one at work who has this problem and I would like to help him out, so I wondered if the answer might be out there before we start looking at his car again.

Perry

[Lockjaw]

In the past when I have had problems like that, I have always cleaned all my connections to the ecu and the various sensors with brake clean and then tried it again. You may also want to check the temp of the coolant with a good thermometer. I am sure the factory ecu will pull some timing out of the thing if the coolant temp is to high.

 

Switch to 180 degree thermostat if you don't already have one in there. Make sure the rad is in good shape, and the fan clutch is not worn out.

[Guest Perry]

Anyone else had this problem? I used to maintain my car well and clean connectors, change the O2 sensor, check for vacuum leaks, verify air meter readings etc.. when I would have this hesitation and a lot of things seemed to help for a while but it would always come back or hesitate intermittently. maybe I'll clean all the connectors on my friends car then fill them with dielectric grease so they won't corrode again to see if that takes care of the problem.

Perry

[Tony240ZT]

Pulled all of my parts for the turbo swap out of the junk yard, luckly most of the components were good. Since my setup is a little custom with the little 2.4L I knew I was going to have to adjust my AFM. With some setups with the AFM there is a hesitation, adjust the AFM to correct the over richness, or lean situation and hesitation goes away. Having an air/fuel ratio meter greatly helps in setting things up.

 

Lots of other possibilities, but in my setup where I've replaced all of the filters, ignition stuff is up to par, etc the AFM is where I direct most of my time, along with the base timing.

[Guest Anonymous]

what do you run for base timing in your turbo motor? The manuel calls for 26 degrees base. This seems like a whole lot to me, since everything else in the world is around six to eight.

-jeremy-

[DAW]

26 degrees base would be to test the knock sensor to see if it can save your engine. That sounds more like total advance that the dist/cas and ecu can put in, in addition to the base (static) advance.

If all connections are clean, on a stock engine the hesitation (poor throttle response) is probably due to a lean condition or inadequate timing advance, or both. I'd recommend some simple tuning tricks which are reversible but should help overall performance regardless. First, check your thermostat (with the meat thermometer borrowed from the kitchen so that you can test instead of actually pulling the thermostat to see the number stamped on it: actual function is better anyway than what it's supposed to be). You want 180, not 195 or 190. If it's not 180, don't go further without making it 180 because that's what signal you want to go to the ecu.

Next, try advancing the timing just a little, maybe 2 to 4 degrees over stock specs.

 

Next, splice a RadioShack #271-1117 680 ohm resistor in one of either wire to the cylinder head temp sensor (use Posi-Twist connectors from auto parts store). This will fool the computer into richenning the mixture just a bit.

If improved by these measures (it will be) but still needs a little more, then tackle the AFM. I wouldn't recommend going there first because it's a bit more involved and if you should loose track of how much you modified the AFM spring tension, it can become a mess. All these measures will help higher end performance too. DAW

[SleeperZ]

I believe the base timing for the L28T is 20 degrees. That's what I've run, and it seems to work well enough without any detonation or hesitation. Of course I'm running a Z31 ECU and MAF, and 13psi intercooooooled boost.

[Lockjaw]

I believe the proper stock timing setting is 24 degree's plus or minus 4. I usually set mine at 20 since I run way more than 13 pounds of intercooled boost.

 

I also am not a fan of putting resistors in line with any of the connections, that is a bandaid fix at best. A ZX with properly functioning efi equip should not need any tinkering.

 

One other thing to check is all the rubber hoses connected to the engine. Since the turbo generates extra heat in the engine compartment, it could create a vaccuum leak.

 

I would not mess wth the AFM unless you have physically changed something about the engine. IE swapped in bigger injectors, or something major like that. Minor changes should not require an AFM adjustment.

 

You could also check the tps and make sure you are getting no voltage fluctuations from the O2 sensor at idle. The car should be in open loop at idle, and if you see voltage fluctuations, a tps adjustment is probably needed.

 

Good luck with it.

[DAW]

OK, so timing is set at 20 degrees at idle with no load on the 7.4:1 c.r. engine and the ECCS system which is computer-managing the timing and changing it with all loads. Put a conventional distributor in a turbo engine (or high compression n/a) and set the timing at 20 degrees BTDC and see what happens. At least you can set/adjust the timing on an L28ET. On most OBD II cars you can twist the distributor back & forth and the computer just ignores you.

 

As to altering resistance in a circuit that uses resistance as the input by which the computer alters the air/fuel ratio it gives the cylinders to burn, it's the same as changing jets in a carburetor; it accomplishes the same thing. It is not the only means of adjusting A/F ratio to get the best performance. Adjustments in fuel pressure, injector size, duration of opening, etc., can all be used to affect A/F but these may be better suited to cover demands due to increased boost or c.r. rather than low speed hesitations on a stock engine (you may also spend $1,000 more than what you needed in order to accomplish exactly what you needed to). Whether the resistance input to the ECU is changed by AFM door spring pressure or swapping thermistors, either air inlet temp or cyl head/water temp, for an alternative one other than the Bosch OEM to send the desired value to the ECU to calibrate the A/F, or adding a specific value of resistance by adding series and/or parallel resistors doesn't much matter, it's the A/F end result which matters. Rather than accept anyone's advice, tip or opinion, the best way is to buy an A/F Monitor and see first what you have that's yielding a certain drivability or performance problem that's typical of over-lean condition, then use it to follow any changes you make to improve performance (by whatever means you choose to do it). If mixture is already rich enough, then try improving throttle response by advancing the timing as the pistons are better protected than if it's lean. DAW