At wit's end

viperdriver · Jan 23, 2014

Good morning. I'm now in month 3 trying to get my '88 Bronco II running properly. Background: Found this XLT for sale, buried halfway up the wheels in the ground, had been sitting 4 years, half a tank of fuel remained. Amazingly it started and drove out of the hole with just a battery and aired up the tires. Had engine rebuilt (not reputable shop as it turns out), transmission rebuilt, headers and high flow catalytic and resonators... have replaced pretty much every part on the engine from alternator, A/C, TPI, coil, distributor, computer and more. I got it running but rough... tweaked for a month, took to local mechanic and he gave up. Took to a more reputable mechanic in Boyd, TX and he had it for two weeks while I was on a business trip. $400 later, swapped out my new parts for other new parts, have the same issue which goes something like this: starts fine, while cold: sounds good, idles great, drives normally through just above idle throttle. Also, runs smoothly when floored. Example. I accelerate to 55mph, set the cruise and it runs great until the first hill/load on the engine. It then misses and loses all power until the throttle is near full at which time it revs up, runs smoothly to 55 and then loses power again. If I leave it on cruise it goes from 55 to 45, loses power, then when full throttle accelerates back to 55 at which point it again loses all power... over and over. I don't think there are any parts not new! I'm wondering if there is a 2.9L expert in the North Texas region who I can take this too or if some of you have had a similar experience/aha on this.

ab_slack · Jan 23, 2014

How about getting some codes out of it?

MAXIMUSPRIME · Jan 23, 2014

I have a similar problem on mine. From what I have read, likely suspects are throttle position sensor (and also the connector), fuel pump/filter, O2 sensors.

Earl43P · Jan 23, 2014

Your symptoms indicate that it runs good in open loop (when not reading the sensors' signals).

You have an Emissions system problem, could be the O2 sensor, MAP, TPS, ECT, etc.

See here for testing these sensors: http://oldfuelinjection.com/

If the O2 sensor isn't new I'd replace it, after verifying the other sensors are good.

viperdriver · Jan 23, 2014

Thank you.

Thank you. I've replaced the TPI, O2 Sensors, fuel pumps and filter. Is there anything special about installing the TPI or just slide it in, tighten the fasteners and plug it in? Agree on the open loop analysis. Although it has been replaced, is it likely the computer, under the dash, could be the culprit?

shane96ranger · Jan 23, 2014

viperdriver said:
Thank you. I've replaced the TPI, O2 Sensors, fuel pumps and filter. Is there anything special about installing the TPI or just slide it in, tighten the fasteners and plug it in? Agree on the open loop analysis. Although it has been replaced, is it likely the computer, under the dash, could be the culprit?

With the engine off, key forward - you can measure the voltage coming out of the green wire. In the idle position you should be from .7 to 1.0 VDC. At wide open throttle you should measure 4.7 to 5.0 VDC. You can loosen the screws and move it slightly if it is off. I've seen people elongate the screw holes on the TPS to get it right as well.

Being brand new, there's a good chance it is OK though.

ab_slack · Jan 23, 2014

A quick check of O2 sensor is to simply unplug it. If it runs better you are getting faulty info from the sensor.

I understand the frustration with shops. If it isn't something they seen they tend to not do so well. The rates they charge, typically around here $100 to $200 per hour it is like their time is too valuable to troubleshoot. Or maybe more correctly they fear they spend time troubleshooting and end up having nothing to show and can't justify a big bill for which they didn't resolve the issue.

Every time I think "I have no idea, let me bring it to a shop cause I can't figure it out" the shops have done no better. I consider myself a hack at this stuff which makes we wonder about the people in the shop.

And there are evidently very good professionals out there who know their stuff. A number contribute here and are immensely helpful. I guess it is just hard to find them or management constraints put on them doesn't allow them to really do the work.

RonD · Jan 23, 2014

I know it's too late but you shouldn't swap sensors to "find" a problem.

Engines are still just engines, contrary to popular belief the computer and sensors just make them more reliable, so less carb tweaking, point filing and timing setting are required, lol.

So spark, fuel, mechanical.

Engine starts and runs at idle OK, and lower rpms are OK.
But issues arise when more RPMs are needed
Could be not enough fuel is being delivered to engine at sustained higher rpms, fuel tank and lines had been sitting(4 years), could be there is debris flaking off and clogging new filter(I assume new filter) or debris in tank is clogging screen.

Fuel pump(s) don't run all the time, when at Full Throttle(WOT) pump(s) are turned on, so if fuel system was clogged up turning on pump(s) might get back enough pressure to get RPMs up again, then when you go back to 3/4 throttle(pumps off) pressure starts dropping again.
I would get or rent a fuel pressure gauge and do a long term pressure test at 2,500rpms, it should stay at 32-40psi.

Another test which you could do right now would be to remove or loosen the gas cap.
The EVAP system is used to draw fuel vapor from the fuel tank, gas caps are often vented to prevent too much negative pressure from building up in the tank, too much negative pressure can limit fuel flow out of the tank, if gas cap is loose or off then no pressure difference can occur.
So you could just have a bad gas cap, lol, do not replace it test it

ab_slack · Jan 23, 2014

First off Ron always has good info!!

And he makes a good point about the wide open throttle explaining the difference.

I would note that, even at higher RPM the engine still won't draw the same fuel as it will under load at that RPM so even if that shows good parked, it might not be definitive.

RonD said:
gas caps are often vented to prevent too much negative pressure from building up in the tank, too much negative pressure can limit fuel flow out of the tank, if gas cap is loose or off then no pressure difference can occur.
So you could just have a bad gas cap, lol, do not replace it test it

And yes sometimes it is the most stupid things.

We had a mechanic tell us the Jeep we have has major engine problems and it wasn't worth fixing (it was pretty beat up otherwise). They said it was something mechanical at the bottom end.

To me it seemed like a basic ignition miss. Before bringing it to the shop we had replaced cap, rotor, wires and plugs (which were horrid) and still had miss. We gave up on our work, specifically asked the shop to review our ignition work cause it seemed like ignition miss, verify we didn't get anything wrong and if that was okay to find the problem.

To make a long story short, after the shop told us this we weren't optimistic. We poked at it without luck. Eventually we went back over our work and discovered that when we replaced cap and rotor we had accidentally bent the electrode on the rotor that added gap. Replaced the rotor again and it runs like a champ.

viperdriver · Jan 23, 2014

Checked the TPI

You guys are great, thanks so much. One of you mentioned being a hack... that would be two of us. You are right about not replacing all the parts as I did in this case... lesson learned.

I checked the TPI and the full-throttle voltage was around 4.05... I skewed it enough to get it up to 4.72, will need to drill/get a different unit to go higher.

Drove the Bronco and no difference, seems a bit tighter on the shutdown vs power back gap, but still really bad... everything else is new including a new distributor as of last week.

Going to loosen the gas cap and try this all again. Fuel pressures were tested and never got above 26-31 psi... I guess that could be the gas cap. Back with further report shortly. Thanks again, really enjoying having the input.

viperdriver · Jan 23, 2014

Gentlemen, we have a winner. Un#$$#%%$ believable!!! Loosened the gas cap, cruised to a nearby town, rough for a few moments so I took the cap completely off for the return and it purrs down the road. You made my day, just can't believe the months of trying to figure this out. This was not a replacement gas cap! I'm guessing they parked this Bronco in the woods, hoping it would turn in to dirt, because they figured it was "unfixable." Probably put on a new gas cap and never figured it out. LOL Have a great day!

RonD · Jan 23, 2014

TPS(TPI?) voltage is fine as long as closed throttle is below 1v(and above .6v) and WOT voltage is above 4.5v
Computer "learns" TPS voltage as you are driving.
Unhook battery for 5 minutes after adjusting or cleaning any sensor or control.
This will make computer relearn.

Fuel pressure, if it is maintained, should be OK at average 28psi but that is a bit low, so as ab_slack mentioned, if could be too low when under load, so static pressure test can be misleading.
What is the status of the Fuel Pressure Regulator(FPR)?

EDIT:

New gas cap, cool.
Could also be an issue with the EVAP system, venting, but at least you know where to look now.

viperdriver · Jan 23, 2014

OK, phase II. No change to anything, went out again and it is back to it's old self. I'm going to give it a rest, will order a vented fuel cap and then start through the system again.

ab_slack · Jan 23, 2014

I failed emissions once on my BII cause they check fuel caps. It didn't affect operation. Either I couldn't locate one at parts store or the emission rules in the state required me to get a factory gas cap so went to the dealer and was shocked by the price, it was like...$9.00. I didn't think it was possible to get something for under $30 from a dealer.

Don't forget what Ron said. The gas cap is a big clue, but there could be an issue in the evap system. And I think potentially fuel pump/filter? Maybe pulling the cap just improved the situation.

But now you have some proof that it can run well and that helps a great deal with the mental attitude when you doubt things like that rebuild and suspect something too deep for you to find exists.

viperdriver · Jan 23, 2014

OK, I'll need to get up to speed on what to check on the Evap system... off to a conference for a couple days, will hit it hard again on Sunday. Thanks for the advice. Really ran poorly on the second outing, amazing how different it can with nothing at all changed.

At wit's end

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